Why is number of bits always(?) a power of two?

Question

We have 8-bit, 16-bit, 32-bit and 64-bit hardware architectures and operating systems. But not, say, 42-bit or 69-bit ones.

Why? Is it something fundamental that makes 2

Answer 1

Partially, it's a matter of addressing. Having N bits of address allows you to address 2^N bits of memory at most, and the designers of hardware prefer to utilize the most of this capability. So, you can use 3 bits to address 8-bit bus etc...