Cusp -is: a pointed process; sometimes at the margin of a wing.
When the sun is passing from one sign to another, the period is known as a cusp.
There is symmetry about the initial line and a cusp at the origin.
The stealthy boat crept on till midnight; now and then from the cusp of a bay floated out the faint cry of a quail.
The ornaments of the cusp might thus be worked ...