In reply to James Rushforth:
Cannon is more angled, like a cannon, and is on the North ridge of Tryfan. There is a better background (mountains / valleys) in pictures taken of the cannon, but less of a sense of precariousness.
It is common to visit both rocks on the same outing, starting at the base of Tryfan and doing the North ridge, down S ridge of Tryfan and then onto Glyder fach either via Bristly ridge or the Miners path.