A one-way switch controls a light from one position; a two-way arrangement allows control from two positions. The simplified diagram shows a switch interrupting the line conductor in one circuit.

This is a connection model, not a replacement procedure. A working switch, a circuit label or a lamp going out cannot establish safe isolation. Keep fittings closed unless a competent person has established safe access, secure isolation and the necessary tests. Read Safe Isolation for the safeguards and their limits.