Inglefield Clip

The Inglefield clip (also known as a sister clip and a Brummel hook) is a clip for joining a flag or ensign quickly, easily and securely to flag halyards so that the flag can be hoisted.

Each clip resembles a link of chain, with a split through one side. The edges of this split are chamfered so that the clips can be engaged or disengaged, but only if they are carefully aligned by hand. When pulled tight, the links are securely fastened. There are no moving parts to the link.
https://en.wikipedia.org/wiki/Inglefield_clip