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.
It is typically constructed from a short piece or tuft of yarn placed in the free air stream where it is visible to the pilot.  In closed-cockpit aircraft, it is usually taped to the aircraft canopy. It may also be mounted on the aircraft's nose, either directly on the skin, or elevated on a mast, in which case it may also be fitted with a small paper cone at the trailing end.  They are commonly used on gliders, but may also be found on jet aircraft (especially fighters), ultralight aircraft, light-sport aircraft, autogyros,  airplanes and helicopters. Its usefulness on airplanes with a tractor configuration (single propeller at the nose) is limited because the propeller creates turbulence and the spiral slipstream displaces the string to one side. 
The yaw string is considered a primary flight reference instrument on gliders, which must be flown with near zero sideslip angle to reduce drag as much as possible. It is valued for its high sensitivity, and the fact that it is presented in a head-up display. Even the most sophisticated modern racing sailplanes are fitted with yaw strings by their pilots, who reference them constantly throughout the flight.
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.  Wilbur Wright is credited with its invention, having applied it concurrently with the movable rudder invented by his brother Orville in October 1902,  although others may have used it before. Glenn Curtiss also used it on his early airplanes.
In flight, the rule to remember is simple: step on the head of the yaw string, the head is the front of the string, where the string is taped to the canopy. If the head of the yaw string is to the right of the yaw string tail, then the pilot needs to apply right rudder pressure. If the head of the yaw string is to the left of the yaw string tail, then the pilot should apply left rudder pressure. Or think of the attached point of the yaw string as an arrow head pointing to the rudder pedal needing the application of pressure. According to Helmut Reichmann, "...rudder against the string and/or aileron toward the string. In a spin the string always points to the inside." To center the ball on an inclinometer, apply rudder pressure on the side to which the ball is deflected. Use the simple rule, “step on the ball,” to remember which rudder pedal to press.     
Yaw strings are also fitted to the Lockheed U-2 high-altitude surveillance aircraft.  A flat spin, caused by excessive sideslip even in level flight, happens much more easily at high altitudes. Some light twin-engine airplane pilots place yaw strings on their aircraft to help maintain control in the event of an engine failure, because the slip-skid indicator ball is not accurate in this case.  In a multiengine airplane with an inoperative engine, the centered ball is no longer the indicator of zero sideslip due to asymmetrical thrust. The yaw string is the only flight instrument that will directly tell the pilot the flight conditions for zero sideslip. 
Yaw strings are also used on some (especially smaller) helicopters.
A variation of the yaw string is the side string, used in gliders for a determination of the angle of attack. In this way the best L/D speed, the best thermalling speed and the stall speed can be measured independently of all parameters like air speed, all up weight, acceleration due to turning or stick movements, and gusts. 
Investigations of the side string and on its use in glider flight were performed from the Akaflieg Cologne. 
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The slip string is commonly referred to as a "yaw string", even though it measures the slip angle, not the yaw angle (i.e. heading)....
For all the progress that has been made in a century of aviation, the Ventur 2 still has a "yaw string" attached where the pilot can see it. As does the Wright glider. This was the first flight instrument....
[Wilbur Wright] responded, "Yes...we will install a short piece of string out front where we can see it. This string will tell us all we need to know!" (paraphrased) Wilbur had just invented the yaw string — the first aircraft instrument.
A yaw string (like those used on sailplanes) above each canopy silently admonishes those who allow the aircraft to slip or skid when maneuvering.
You will see that with the wings level the yaw string does not lay along the centerline, yet the ball is in the center...
This article incorporates public domain material from the United States Government document: " Instrument Flying Handbook ".