Notes on data
Long course and short course
Prior to May 1, 1957, long course and short course records were not kept separately. For each distance and stroke, a record could be set in either a 25m pool or a 50m pool. Most records were set in 25m pools, and in my data, following the FINA Swimming Annuals, these records are listed as short course records. (The Wikipedia lists some of these records as long course.)
Yard pools
Up to the early 1960s (as far as I can tell) records could be set in pools measured in yards; a swim of 110 yards (about 100.6m) in a 50 yard pool, for example, could be recognized as a record in a 100m event.
Long course 50 free
The first world record in the long course 50 free that was recognized by FINA was swum in August 1986. Times before then are world best times, as reported in the FINA Swimming Annuals.
Backstroke
Starting March 3, 1991, no hand touch was required at a turn in backstroke events.
Long course 50 backstroke
The first world record in the long course 50 backstroke that was recognized by FINA was swum in June 1999. Times before then are world best times, as reported in the FINA Swimming Annuals.
Long course 50 fly
The first world record in the long course 50 fly that was recognized by FINA was swum in June 1999. Times before then are world best times, as reported in the FINA Swimming Annuals.
Long course 50 breaststroke
The first world record in the long course 50 breaststroke that was recognized by FINA was swum in August 1998. Times before then are world best times, as reported in the FINA Swimming Annuals.
Short course 50 backstroke, breaststroke, and fly
FINA started recognizing world record times in the short course 50 backstroke, 50 breaststroke, and 50 fly on October 31, 1994. It started recognizing short course records in other events on March 31, 1991. Times before then are world best times, as reported in the FINA Swimming Annuals.
Breaststroke and butterfly
Starting January 1, 1953, breaststroke and butterfly were recognized as different strokes. Previously, using the stroke now classified as "butterfly" was allowed in a breast stroke event. Consequently, the record times for breast stroke events increased after this date.
Non-textile bodysuits
From February 2008 until December 31, 2009, records could be set by swimmers wearing bodysuits or suits made of polyurethane or other non-textile materials.
Mixed relays
My data does not include mixed relays, which FINA started including in its records in 2013.