Intel processors have changed in many ways, but each change has attempted to be backward compatbile with previous versions.

A significant aspect of a processor is the set of instructions it is designed to execute.

Backward compatibility means that the instruction set of a newer processor includes the instruction set of previous versions and they have the same effect (in some sense).

Instead of identifying different instruction sets with the different versions of the processors, the instruction set of the Intel processors is referred to (by Intel) as the IA32 instruction set. (Intel Architecture - 32 bit)