A chip log, also called common log, or ship log, is a navigation tool used by mariners to estimate the speed of a vessel through water. The name of the unit knot, for nautical mile per hour, was derived from this method of measurement. A chip log consists of a wooden board attached to a line (the log-line). The log-line has a number of knots tied in it at uniform spacings. The log-line is wound on a reel to allow it to be paid out easily in use. Originally, the distance between marks was 7 fathoms or 42 feet used with a sandglass with a 30 second running time. Later refinements in the length of the nautical mile caused the distance between knots to be changed. Eventually, the distance was set to 47 feet, 3 inches for a standard glass of 28 seconds.