A Shared Side

Two triangles often sit next to each other and share a side. That side is a side of both triangles at once, and it is equal to itself. In a proof the reason for writing it is the Reflexive property: anything equals itself.

Here △ABC and △ABD share AB. The line looks too obvious to write, but a congruence rule needs three pairs of sides, and the shared side is one of the pairs. Spotting it is often the step that unlocks a proof.