In a 64 bit register, e.g., RAX, AL refers to the lowest 8 bits [0-7] and AH refers to the next 8 bits [8-15].

Together, AX refers to bits [0-15]. EAX refers to [0-31].

It's counterintuitive (or at least, inconsistent) that we have a name for bits [8-15] but not for [16-31] or [32-63]. My fuzzy understanding is that this came about from legacy decisions.

This page has a helpful visualization at the top: https://www.cs.uaf.edu/2017/fall/cs301/lecture/09_11_registe...