If you collect a mixture of sizes, sometimes
there's room to store the smaller
machines inside the bigger ones.
Ah, so that's where I left a couple of HP41s -- inside a Unibus expansion
box :-)
Or :
If you can lose your screwdriver inside it, it's a micro
If you can lose your 'scope inside it, it's a mini
If you an lose yourself inside it, it's a mainframe.
-tony