Back in the late 1600s, a keynote referred to the first note – and basis of – a musical key, like C major. Today, musicians more commonly call this the tonic. But the concept of a keynote plays well as a metaphor. The main idea of a speech or text acts like a keynote, sounded at the beginning, resolved to at the end, and setting the prevailing tone throughout. The Oxford Dictionary English (OED) dates this figurative usage to 1763.