The yaw string dates from the earliest days of aviation, and actually was the first flight instrument. The Wright Brothers used a yaw string on their 1902 glider tied on their front mounted elevator.
The yaw string, also known as a slip string, is a simple device for indicating a slip or skid in an aircraft in flight. It performs the same function as the slip-skid indicator ball, but is more sensitive, and does not require the pilot to look down at the instrument panel. Technically, it measures sideslip angle, not yaw angle, but this indicates how the aircraft must be yawed to return the sideslip angle to zero.