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A diamond turn is a kind of turning manoeuvre used by motor vehicles at intersections.
This explanation is for countries where vehicles drive on the left side of the road (driver sits on the right inside the vehicle). For example, the UK or Australia. For other countries, the description would be reversed.
At a four-way intersection, if two drivers approach from opposite directions, and each of them wishes to turn right, and if there is room to do so at the same time, they should drive through the intersection from the entering road to the departing road, so that the left side of each vehicle passes the left side of the other vehicle, and the actual vehicle paths do not intersect.
This is nowadays the standard type of right turn manoeuvre at a four-way intersection. In particular, at signalised intersections, it enables a traffic signal phase to allow traffic approaching the intersection from two opposing directions, to both turn right simultaneously without conflict with vehicles coming from the opposite direction and also turning right.
The diamond turn is distinguished from the manoeuvre historically required, where drivers simultaneously turning right from opposing directions were required to proceed straight ahead two thirds of the way across the square formed by the two intersecting streets, and then turn right, which resulted in the two vehicles looping past one another with their right (driver) sides adjacent, as if there was a pole or a roundabout or a policeman standing in the middle of the intersection which had to be driven around. This type of turn was eliminated because it was unsuitable for signalised intersections with simultaneous dedicated right turn green signal phases for more than one approaching street.
The introduction of the diamond turn in Australia was one of the motivations for the elimination of the silent cop.
A diagram showing the two types of turns appear at. However the description of these alternative manoeuvres as "diagonal turns" and "diamond turns" appears to be swapped.