Herein lies a comprehensive outline of the differences between MTGO Pauper and paper Pauper. This info will probably be more helpful in this dedicated thread than being buried elsewhere. All future updates and corrections will be made in this thread.
Pauper Rarity Breakdown:
Except for Arabian Nights, Antiquities, The Dark, Fallen Empires, Chronicles, Homelands, & Alliances, all other boosters prior to the introduction of mythic rares (even Alpha, if my source is correct) contained 15 cards at a rarity breakdown of 11-3-1. (11 commons, 3 uncommons, & 1 rare per booster) Most of those exceptions, however, only contained 8 cards and had a rarity breakdown of 6-2. (6 commons* & 2 uncommons* per booster) Alliances had a breakdown of 8-3-1, while Chronicles had a breakdown of 9-3. Legends had a more normal breakdown of 11-3-1.
* these terms are ambiguous and are discussed further in the next section
Most sets follow a standard for rarity distribution (11-3-1: 11 cards from one sheet of 121 cards, 3 from another, and 1 from a third), which smoothly fit into a standard for card rarity (11 commons, 3 commons, & 1 rare per booster). The terms 'common', 'uncommon', and 'rare' are defined by this standard rarity distribution. For non-standard sets, actual rarity isn't so easily correlated with rarity distribution: in order to use those terms in the same context in which they're defined (i.e. for them to have the same meaning), we must first translate into a standard. If this isn't done, a card from the 'common' sheet or 'uncommon' sheet of a non-standard (6-2) set might be considered 'common' or 'uncommon' on the basis of what sheet it was printed on, but those terms wouldn't match the standard that we mean with them. Using the same words to mean different things, especially when those things are similar, makes understanding quite difficult. For the sake of clarity, let's briefly use other terms: those sheets could just as easily be titled 'normal' and 'special'.
Since the rarity of these 'normal' cards is defined by their taking 6 slots in an 8-slot booster, it's clear that they aren't distributed the same as 'common' cards, which are defined as taking 11 slots in a 15-slot booster. Both terms are meant to express how rare a card is, or how often the card appears in a booster. By comparing the actual rarity of each card, we can see how cards from non-standard sets fit into our standard classification of 'common', 'uncommon', and 'rare'.
Note: The terms 'common' and 'uncommon' and abbreviations such as 'C1' will continue to be used for early sets because using a new format such as N1 (printed once on the normal sheet) or S3 (printed 3 times on the special sheet) doesn't clarify the issue well enough to be intuitive considering how ingrained the traditional formatting is.
Many early expansions used 2 sheets rather than 3, but often had a greater number of distinct rarities across all cards than later expansions due to some cards being printed multiple times per sheet. The terms C1, C4, U3 and such represent which sheet the cards were printed on (Common or Uncommon) and how many appeared on the sheet. (1, 4, or 3, respectively) For most of these sets, both sheets contained 121 cards each, organized into 11 columns and 11 rows before being cut into individual cards.
As an example, Arabian Nights had 32 U2 cards (32 cards printed twice each on the uncommon sheet) and 19 U3 cards (each printed three times on the uncommon sheet). Each sheet had 121 cards in total, which can be checked with a little math: 32 * 2 = 64, 19 * 3 = 57, and 64 + 57 = 121. The same can be done for the common* sheet as well as any other set.
The average number of any card appearing in a booster pack depends on how often that card appears on a sheet as well as how often cards from that sheet appear in a booster. In the case of Arabian Nights, the boosters contained 8 cards: 6 from the common sheet and 2 from the uncommon sheet. A U2 card, which appeared twice on the uncommon sheet, would be found twice per 121 cards (since each uncommon sheet had that many cards). Math shows us that 2/121 = 0.0165, or 1.65%. However, because there were 2 uncommons per booster, that number is doubled: (2 / 121) * 2 = 0.0331, or 3.31% of the cards per Arabian Nights booster will be any particular U2 card on average. (assuming complete randomization) The same math can be applied to any card in any set, and the results are shown below.
Arabian Nights (78 total cards) Breakdown: 6-2 | Cards per sheet: 121 | Released: December 1993
32 U2 (uncommons) . . . . . each 3.31% per booster
19 U3 (uncommons) . . . . . each 4.96% per booster
1 C1 (basic Mountain - uncommon, as prevalent as U3 cards) . . . 4.96% per booster
16 C4 . . . . . . . . . . . . . . . . . . each 19.83% per booster
9 C5 . . . . . . . . . . . . . . . . . . . each 24.8% per booster
1 C11 (Desert) . . . . . . . . . . . 54.55% per booster Note: Arabian Nights had a much flatter rarity distribution than most sets, with 2 main groups of rarities and no cards even as rare as the uncommons of later sets. Whoever entered the data for this set into Gatherer listed the U2s as rares, while many rarer cards from other sets were listed as uncommons. It seems likely that the method used to determine Gatherer rarity involved some amount of guesswork when sets were first added.
Antiquities (85 total cards) Breakdown: 6-2 | Cards per sheet: 121 | Released: March 1994
20 U1 (rares) . . . . . . . . . each 1.65% per booster
4 U2 (uncommons). . . . each 3.31% per booster
29 U3 (uncommons) . . . each 4.96% per booster
2 C1 (uncommons, just as prevalent as U3 cards) . . . each 4.96% per booster
2 C2* (commons) . . . . . each 9.92% per booster
25 C4 . . . . . . . . . . . . . . . each 19.83% per booster
1 C5^ (Urza's Tower) . . . 24.8% per booster
2 C6^ . . . . . . . . . . . . . . . each 29.76% per booster Notes:
* Mishra's Factory are Strip Mine were printed 3 times on the uncommon sheet, but are unique in that they were also printed once on the common sheet, and are thus neither U3 nor C1, but rather the rarity equivalent of both combined: either C2 or U6.
^ Urza's Tower, Mine, and Power Plant were all printed with 4 separate art versions. The Tower had 1 of those versions duplicated on the common sheet while the two other cards each had 2 versions duplicated, so Urza's Tower is C5 while the others are C6.
Legends (310 total cards) Breakdown: 11-3-1 | Cards per sheet: 121 | Released: June 1994
121 R1 . . . each 0.83% per booster
107 U1 . . . each 2.48% per booster
7 U2 . . . . . each 4.96% per booster
29 C1 . . . . each 9.09% per booster
46 C2 . . . . each 18.18% per booster
The Dark (119 total cards) Breakdown: 6-2 | Cards per sheet: 121 | Released: August 1994
35 U1 (rares) . . . . . . . . . each 1.65% per booster
43 U2 (uncommons) . . . each 3.31% per booster
1 C1 (Maze of Ith - uncommon, as prevalent as uncommons in other sets) . . . 4.96% per booster
40 C3 . . . . . . . . . . . . . . . each 14.88% per booster
Fallen Empires (102 total cards) Breakdown: 6-2 | Cards per sheet: 121 | Released: November 1994
36 U1 (rares) . . . . . . . . . each 1.65% per booster
5 U2 (uncommons) . . . . each 3.31% per booster
24 U3 (uncommons) . . . each 4.96% per booster
2 C1^ (uncommons, just as prevalent as U3s) . . . . 4.96% per booster
1 C3* (Farrel's Zealot, common) . . . 14.88% per booster
19 C3 . . . . . . . . . . . . . . . each 14.88% per booster
15 C4 . . . . . . . . . . . . . . . each 19.83% per booster Notes:
^ The 2 C1s are Farrel's Mantle and Delif's Cone. Gatherer lists the latter as common for an unknown reason.
* Farrel's Zealot was printed twice on the common sheet and 3 times on the uncommon sheet, so is neither C2 nor U3, but rather a combination of both that has the equivalent rarity of C3.
Ice Age (373 total cards) Breakdown: 11-3-1 | Cards per sheet: 121 | Released: June 1995
121 R1 . . . each 0.83% per booster
121 U1 . . . each 2.48% per booster
121 C1 . . . each 9.09% per booster Note: Rares, uncommons, and commons add up to a total of 363 cards. The remaining 10 cards are the 5 basic lands and 5 basic snow lands.
Chronicles (116 total cards) Breakdown: 9-3 | Cards per sheet: 121 | Released: June 1995
46 U1 (rares) . . . . . . . . . each 2.48% per booster
25 U3 (uncommons) . . . each 7.44% per booster
5 C1 (uncommons) . . . . each 7.44% per booster
7 C2 . . . . . . . . . . . . . . . . each 14.88% per booster
30 C3 . . . . . . . . . . . . . . . each 22.31% per booster
3 C4 . . . . . . . . . . . . . . . . each 29.75% per booster Note: These U1s would normally be considered uncommons from a static percentage-based rarity perspective, but this set seems to showcase that relative rarity between rarities should be taken into account - these boosters contained more cards at each rarity, but the relative difference between those rarities was quite normal.
Homelands (115 total cards) Breakdown: 6-2 | Cards per sheet: 121 | Released: October 1995
43 U1 (rares) . . . . . . . . . each 1.65% per booster
26 U3 (uncommons) . . . each 4.96% per booster
21 C1 (uncommons, just as prevalent as U3s) . . . each 4.96% per booster
25 C4 . . . . . . . . . . . . . . . each 19.84% per booster Note: Each C4 is technically 2 C2 versions of the same card with different art.
Alliances (144 total cards) Breakdown: 8-3-1 | Cards per sheet: 110 | Released: 10 June 1996
46 R2 . . . . . . . . . . . . . . . each 0.91% per booster
3 R6 (uncommons) . . . . each 2.73% per booster
40 U2 (uncommons) . . . each 5.45% per booster
5 U6 (commons) . . . . . . each 16.36% per booster
40 C2 . . . . . . . . . . . . . . . each 14.54% per booster
10 C3 . . . . . . . . . . . . . . . each 21.82% per booster
While Gatherer seems to (mostly) display rarities based on which sheet cards were printed on, and ignoring how often they were printed on it, Magiccards.info seems to display rarities based on how rare the cards were, regardless of how they were printed. (see the 'Contestable Cards' section below for the exceptions to this organization) It seems the rarity thresholds are as follows (see the above section for the math that results in these values):
<= 1.65%: Rare
2.48% - 7.44%: Uncommon
With this as a basis for rarity, whether a card was printed on a specific card sheet or another doesn't matter. Only the card's rarity (as a percentage) affects it's status as common, uncommon, or rare. Unless otherwise noted, this is how cards will be referred to here.
Note: The practical differences between this system and Gatherer rarities includes ~20 cards total, with Merchant Scroll, Mishra's Factory, and Strip Mine being the only notable cards. See the 'Contestable Cards' section below for a more detailed explanation and complete list of cards whose rarities are different depending on the system used.
It's important to remember that C1 simply means 'printed on the common sheet 1 time'. For sets with a breakdown of 6-2, (6 cards from the common sheet and 2 from the uncommon sheet) C1 cards had the same rarity as U3 cards, (4.96%) making them uncommons. Most other breakdowns put C1 cards at a lower rarity, making those cards commons.
C1s from Homelands (uncommons) - 4.96% per booster
It seems that Gatherer lists these as commons because they were printed on the 'common' sheet of the print-run, even though they have the same rarity as U3 (uncommon) cards.
C1s with Multiple Versions (commons) - 9.92% per booster or higher
Though each version (different art) is technically uncommon for collecting purposes, these 'cards' are considered to be C2, C3, or C4 depending on how many versions there are, so are common for rarity purposes.
Without sanctioned support, the metagame of paper pauper is difficult to determine. In practice, it may reflect MTGO pauper's metagame, with the exception of commons unavailable there. These are the commons that were never made common for MTGO:
MTGO has none of the cards from sets prior to Mirage, except for those available through the Masters Editions and other specialty sets. Fifth and Sixth Edition (printed after Mirage) and a few recent sets were never added to MTGO either. Additionally, select cards from Conspiracy 1 & 2 and Commander 2016 are available via treasure chests.
Sets never released online:
Duel Decks: Zendikar vs Eldrazi
While the better known and more powerful commons of old have found their way to MTGO at higher rarities, there are many other commons that were never made available again at any rarity, and thus aren't available on MTGO. Of those, there are very few, (if any) standout cards that would be played if legal online. Most have been outclassed:
See the 'contestable cards' section (point #3) below for 15 additional cards which aren't available online as commons, but are sometimes considered as such in paper. They aren't included here because they aren't common from a percentage-based rarity perspective.
Though playing with physical cards, many decide to incorporate rarities from both paper and online sets. These cards lack a physical printing at a common rarity, appearing as commons solely via MTGO-only sets:
Because there's disagreement on the rarity of some cards in these early sets, understanding these differences helps us determine which are legal for our particular environment. Outside of simply abiding by an authority's rarity system and ban list (such as MTGO, Gatherer, or a more local authority as seen in Card Kingdom's Rags to Riches tournaments), there are two main approaches to determining rarity for early sets. One is the percentage-based rarity system, and the other is the common-sheet rarity system - both are mentioned in the opening section here. Points 1-3 below address the relevant older cards with respect to these systems and Gatherer, while the others deal more directly with the differences between paper and MTGO Pauper:
1. Listed as commons on gatherer, but only appeared as frequently as other uncommons:
These had the same rarity (percentage-wise) as other uncommons from the same sets. They seem to be listed as commons on Gatherer due to being printed on the 'common' sheet of their time. See the 'History and Set Information' section above for an explanation of why such a detail is contentious.
2. Listed as uncommons on gatherer, but appeared more frequently than other commons:
These were at least as common (percentage-based rarity) than other commons from similar sets. Gatherer and other sites such as magiccards.info seem to treat each version of these two (4 versions each) separately as far as rarity is concerned, rather than combining all versions as they do for others. For example, neither Hymn to Tourach (4 versions) nor High Tide (3 versions) would be considered common via this method. Both cards were unique in that they were printed 3 times on the uncommon sheet AND once on the common sheet - this may be an explanation for how the mistake originated.
3. C1 'commons' not recognized as such on gatherer:
These could be considered common on the basis that they were printed on the 'common' sheet of their time, regardless of how frequently they could be found in booster packs. Since this "common sheet" criterion seems to be one used for Gatherer in part 3, these cards represent an inconsistency because they were also printed on the 'common' sheet. Note that except for those cards listed in part 4 above, none of these were actually as common as other commons.
4. Arbitrary 'Commons':
While the vast majority of commons were released in draftable sets with some kind of non-flat rarity distribution, a few were 'downshifted' (that is, their rarity symbol was made black) without actually appearing more often than any of the other cards in their set. As a comparison, the FTV sets include each card as a 'mythic rare' even though each set has no true rarity. Suppose that those same sets included each card as a 'common' instead, and it becomes obvious that rarity assigned within non-random sets is arbitrary. These cards can only be considered common via this arbitrary method:
Note: Commander sets, while not randomized, do incorporate a consistent rarity scheme which correctly places cards within standard rarity thresholds.
5. Only introduced at common through MTGO: (see the 'MTGO rarity downshifts to common' list above)
Paper players have the liberty of allowing all commons (whether released online or normally) if desired.
6. Banned in MTGO:
Paper pauper isn't explicitly supported by Wizards, so no official ban list technically exists for it. Some use the online pauper ban list. Some don't use any list. Some use a custom list.
7. Powerful cards absent from MTGO banlist:
For those who use MTGO's banlist for paper Pauper, a number of additional cards that mostly aren't available as commons on MTGO are strong enough to either be banned as well or at least considered for banning. It's difficult to say whether each of these are too powerful without testing, but this list errs on the side of comprehensiveness - any card that's been discussed multiple times as a candidate for banning is listed. The contents of this section are the most subjective of any in the 'Contestable Cards'.
*As the only two Locus lands, banning either Glimmerpost or Cloudpost neutralizes the 8-post mana package. However, since Cloudpost may have some marginal use alone while Glimmerpost is practically useless without its counterpart, banning Glimmerpost rather than Cloudpost improves the format's cardpool slightly.
Peasant, a format identical to Pauper except for the allowance of up to 5 uncommons per deck, went this route with its banlist. (see here) It's interesting to note that as far as commons are concerned, the two banlists are nearly identical. The only other differences are the banning of Delver of Secrets in Peasant rather than Cloud of Faeries, and Invigorate remaining unbanned in Peasant.
A few incorrect points about set history/organization were pointed out and fixed. The main realization of this is that two additional cards from Antiquities were actually quite common - Mishra's Factory and Strip Mine. The details are mentioned above in the rarity breakdown section, but essentially, whether you follow the percentage-based rarity as described here, or you subscribe to the idea that any card printed on the common sheet is common, these two cards are commons. Gatherer, however, doesn't currently list them as commons, probably for the same reason that they were glanced over at first here.
Somewhere in the translation to Gatherer, their rarities were split among each version of art, rather than combined like every other multi-art-version card. Hymn to Tourach, for instance, would not be common if the same was done with it - each of the 4 versions would be considered C1 (uncommon), rather than seen together as C4 (common). In any case, there seems to be no reason for these cards to continue to be considered non-commons, except from a stance that maintains tradition to be more important than rarity.
As these are quite powerful cards and are sure to be contentious in Pauper, they were added to the newly made point #7 of the 'Contestable Cards' section, alongside other similar cards. Gatherer is maintained by Wizards of the Coast, albeit poorly (comments have been temporarily disabled for years now), but it is still the de facto authority for Magic. Simultaneously, Gatherer has mirepresented these cards as uncommons or rares for many years. As always, refer to your local authority or playgroup for a list of banned cards.
In addition to being a new difference between MTGO and Paper Pauper, this is noted because duel deck rarities aren't really rarities - the decks are non-random, with commons and rares appearing at the same frequency. See part 6 of the 'Contestable Cards' section.
1. Only introduced at uncommon through MTGO (see the above list)
2. U2s not recognized as uncommons on magiccards.info:
These are probably mislabeled, as Gatherer lists them as uncommons and they fit the uncommon rarity range described above. They're noted here because magiccards.info is often the go-to card info source rather than Gatherer.
3. Banned in Peasant (taken from mtg-peasant.com) Note: The peasant banlist is maintained independently by a joint French-German rules committee. With no conflicting 'officially sanctioned' banlist (as in Pauper), the committee's banlist is the banlist. There are no differences between online and paper Peasant in this regard, but the list is included here for reference: