Googled it and found this: Pre-1920 – A player generally is required to appear in at least 100 or more games when the schedule was 154 games, and 90 games when the schedule was 140 games. An exception to the rule was made for Ty Cobb in 1914, who appeared in 98 games but had a big lead and was also a favorite of American League President Ban Johnson. 1920–1949 – A player had to appear in 100 games to qualify in the NL; the AL used 100 games from 1920 to 1935, and 400 at-bats from 1936 to 1949. The NL was advised to adopt 400 at-bats for the 1945 season, but National League President Ford Frick refused, feeling that 100 games should stand for the benefit of catchers and injured players. 1950–1956 – A player needed 2.6 at-bats per team game originally scheduled. (With the 154-game schedule of the time, that meant a rounded-off 400 at-bats.) From 1951 to 1954, if the player with the highest average in a league failed to meet the minimum at-bat requirement, the remaining at-bats until qualification (e.g., five at-bats, if the player finished the season with 395 at-bats) were hypothetically considered hitless at-bats; if his recalculated batting average still topped the league, he was awarded the title. This standard applied in the AL from 1936 to 1956. 1957 to the present – A player has needed 3.1 plate appearances per team game originally scheduled; thus, players were no longer penalized for walking so frequently, nor did they benefit from walking so rarely. (In 1954, for example, Ted Williams batted .345 but had only 386 ABs, while topping the AL with 136 walks. Williams thus lost the batting title to Cleveland's Bobby Ávila, who hit .341 in 555 ABs.) In the 154-game schedule, the required number of plate appearances was 477, and since the era of the 162-game schedule, the requisite number of plate appearances has been 502. Adjustments to this figure have been made during strike-shortened seasons, such as 1972, 1981, 1994, and 1995. From 1967 to the present, if the player with the highest average in a league fails to meet the minimum plate-appearance requirement, the remaining at-bats until qualification (e.g., five at-bats, if the player finished the season with 497 plate appearances) are hypothetically considered hitless at-bats; if his recalculated batting average still tops the league, he is awarded the title. This is officially Rule 10.22(a), but it is also known as the Tony Gwynn rule because the Padres' player won the batting crown in 1996 with a .353 average on just 498 plate appearances (i.e., he was four shy). Gwynn was awarded the title since he would have led the league even if he'd gone 0-for-4 in those missing plate appearances. His average would have dropped to .349, five points better than second-place Ellis Burks' .344.[20] In 2012, a one-time amendment to the rule was made to disqualify Melky Cabrera from the title. Cabrera requested that he be disqualified after serving a suspension that season for a positive testosterone test. He had batted .346 with 501 plate appearances, and the original rule would have awarded him the title over San Francisco Giants teammate Buster Posey, who won batting .336.