The zone bits of the middle character of a three character memory address could specify an index register, one of many optional features.

The first three bits after the sign specified an index register.

Typical instructions contain a function code, the target (or source) register, an index register, a base register and a displacement field.

Some instruction sets allow more than one index register to be used; in that case additional instruction fields specify which index registers to use.

The third index register was dedicated to pointing at the current procedure's stack frame on the call/return stack.