Once roster limits became a permanent part of Major League Baseball, a need developed to allow teams to temporarily replace injured players without exceeding the limits. In July 1915, the National League created a Disabled List. Teams were permitted to place an injured player on this list for a period of at least ten days. This allowed the team to remove the player from their active roster without having to release him or sell him to another club. They could then add another player to their active roster until the injury had healed. This rule lasted only through the 1916 season, and wasn't revived until 1941. At that time, both leagues adopted a new disabled list rule, with each team being allowed to place two players on a DL as long as it was done before August 1 and the injured player remained on the list for at least 60 days. In 1949, the minimum stay was shortened to 30 days.
A 60 day "emergency" list was added in 1960, which required the permission of the Commissioner to use. Players on the sixty day list do not count against either the 25 or 40 man rosters. In 1966, the required stay on the regular DL was shortened again to 15 days, but a year later was increased to 21 days. A more liberal change was implemented in 1971, with teams now allowed to put a player (pitchers not included) on a 'supplemental" 15 day DL in addition to the two player limit on the 21 day list. In 1973, a maximum of three players could be put on the 21 day list .
Beginning in 1981, there were four disabled lists: the regular 21
list with a 3-player limit, the 15 day "supplemental" list for one
a new "special" 21 day list to be used when the prior two were full,
the "emergency" 60 day list. In 1984, the "special" list was changed to
require a minimum 30 day stay and the supplemental list was increased
allow two players, including one pitcher. In 1990, the rules were
once again. From then on, there were only two DL's, a fifteen day and a
sixty day list. There were no limits on the number of players that
be placed on these lists. However, the 40 man roster had to be full
the sixty day list could be utilized.