The chelys (, ), was a stringed musical instrument, the common lyre of the ancient Greeks, which had a convex back of tortoiseshell or of wood shaped like the shell. The word chelys was used in allusion to the oldest lyre of the Greeks, which was said to have been invented by Hermes. According to the Homeric Hymn to Hermes (475) he was attracted by sounds of music while walking on the banks of the Nile, and found they proceeded from the shell of a tortoise across which were stretched tendons which the wind had set in vibration.