I use often these little home made squares for holding small parts in the mill vice truly perpendicular to a side. I picked the idea in the workshop manual book by GH Thomas.
As this axle box was sawn from a milled bar, I cannot rely on the base to mill the opposite face, hence this handy square.
Zephyrin touched on it.
A corollary to this is an angle plate. Some are very simple with just slots. Some are very complicated with built in V blocks and ledges. I have several up to 8" and down to about 2"...I seem to draw to them like they have some magnetic property or something! In any case, I find them darn handy either mounted to the table directly, or mounted in the vise. Most of mine are orthogonal. There are some out there that have fixed angles on them though.
They're not expensive, and are handy. Make sure the "Square" ones are truly square.
Another form is the cube. I have some of those as well. Again Mount a small part on one side, and you can easily flip the part any old which way around in the vise and work 5 sides with one set up. Damn handy
You could now argue we've come back full circle to 123 blocks …...