size of knob

Well! Never on the first date...

I've come to appreciate the subtle but smart feature of the Williams Optics 2" / 1¼" adapter for the mirror diagonal:

The mirror proper has a thumbscrew to clamp down a 2" eyepiece or the adapter. The thumbscrew itself is large.

The adapter has a thumbscrew to lock in a 1¼" eyepiece. This thumbscrew is smaller.

In the dark, without looking, you know which part you're releasing...

Smart.