More specifically, they're affixed to certain opcodes that require them. There are a number of byte-sized opcodes that do not require a ModRM or SIB byte (altho

More specifically, they're affixed to certain opcodes that require them. There are a number of byte-sized opcodes that do not require a ModRM or SIB byte (although a number of those got gobbled up to make the REX prefix, but that's another story). TL;DR Weeee! Intel machine language is crazy!