If we ask what a cubit is we will discover that it is the distance from the tip of the middle finger to the point of the elbow. But if we take the next step and ask whose middle finger and whose elbow, we will have to admit that we don't know. It was subject to variation since the measurement was related to different people.

The variation is estimated at between 43 to 53 cm so that we may think of a cubit as being approximately 1 1/2 feet.