Originally, FM was allocated a band from 41.2 to 50.4 megahertz (though only 42.6 to 47.5 were actually assigned). Stations were originally given "experimental" call letters (W or K, followed by a number for the geographic area, an X for experimental, and 2 or 3 more letters), and later they were given call letters based on the frequency. (For example, a station on 43.9 MHz in Boston was given the call letters W39B.) (For more detail, see this article.)
In 1945, the FCC moved the FM band to a new range, 88.1-105.9 MHz. This was in concert with a change that allocated the 44-50 megahertz range to a television channel.) This was done gradually; on October 1, 1946 there were some stations on the 41-50-MHz band, others on the 88-106 MHz band, and others using both; as late as January 1, 1948 some stations were still broadcasting in both bands, but during that period all stations were gradually converting to the upper band only. This was subsequently expanded to 88.1-107.9 MHz.