By a frame I mean four orthogonal vectors for which one is timelike and the others are spacelike. In a bypassing some time ago I read something about the basis vectors in a frame representing sticks and and a clock. However, the author also noted that this interpretation was problematic. I would like very much like to read a more elaborate discussion on the issue. Did Einstein have an interpretation, can you explain it, or can you refer me to some good literature on it?