The 101 Common Problem

Starting with Shards of Alara, most large Magic sets have had 101 commons. These sets have mostly been printed on 11 × 11 sheets with room for 121 cards. These numbers actually work out well for foils because 4 variations of each standard basic land can be added to the 101 commons to fill up a sheet. It isn't so obvious how the normal cards are printed.

The normal solution to this problem is to divide the cards into a 60 card group and a 40 card group with one extra card remaining. One sheet is printed with each of card of the 60 card group appearing twice plus the extra card and the other sheet is printed with each card of the 40 card group appearing three times plus the extra card. [1]

Then, in order to ensure each card is printed at the same rate, the sheets are printed in a 3 : 2 ratio. (That is, for every five sheets printed, three will be the sheet with the 60 card group, and two will be the sheet with the 40 card group.) This works to print every card equally except for the extra 101st card which is only printed 5 times for every 6 of any other common. (Oftentimes, this short-printed card is chosen to be a colorless card so as not to upset color balance.)

Sometimes, each sheet just makes up one large print run and each pack gets a few cards from each run. Othertimes (as is the case for modern US printed sets), things are more complicated.

[1] This method of distributing the cards on sheets (and many of its consequences) were discovered by user Medussa on the MTG Salvation forums.