Contraltos are the lowest female singers commonly heard. Unlike their male counterparts (the basses), however, roles for altos are a bit more common, especially in rock musicals. In more traditional pieces, altos are frequently middle-aged leading women, though some of those are mezzos as well. The only role demographic altos have a firm hold over is for women over sixty - which in most shows means grandmothers. In the rare case that there is a female villain in a show, she will probably be an alto.
Before Pam, Jim was with Karen
Before Monica, Chandler was with Janice
Before Jess, Nick was with Reagan
Before Rachel, Ross was with Julie
Before Peyton, Lucas was with Lindsey
Before Erin, Jay was with Gabby
Baritones, like their female counterpart, the mezzos, come in all shapes and sizes. They are common for leading men, villains and character parts. There are even a few ingenues or at least roles for young men in the baritone range. Because it is a range shared by many singers, there are many subtypes of baritone roles. Villains and leading men in heavier pieces are frequently dramatic baritones, whereas character types are comic baritones and younger characters or characters in lighter shows are lyric baritones. Because it is the most common male vocal register, the voice can also have an “everyman” implication and such roles are frequently baritones.