why call it a "key" system?

why call it a "key" system?
Perhaps because a line button on a multi-line phone opened and closed a circuit, it was called a "key."

Or maybe it was called a key because pressing it reminded someone of pressing a piano key or a telegraph key.

At any rate, multi-line phones became known as "key phones," and the relays and wires and other stuff they got connected to was called a "key telephone system," or "KTS" in Bell System lingo.