User blog comment:SpiritedDreaming/RWBY: Attack on Kevin Tower Defense Game/@comment-25936766-20160920122749/@comment-28288412-20160920190633

As with 2 and nd as well as 1 and st. It's easy to tell if you use the pronunciation of the numbers. Eleven, twelve and then the "teens" all use th. While one, two and three use their own respective (suffix?). This holds true with any number containing the pronunciation in those, for example 33 which is pronounced thirty three and thus thirty third.