The leading edge is located at the bottom of the clubface. Precisely, it corresponds to the border of the sole as it meets the clubface, or in other words, the area where the clubface meets the sole. It is the area of the club head that is meant to penetrate the ground first when striking a ball, with an iron or a wedge for example.
A ball struck by the leading edge (instead of the sweet spot) is called a thin shot and will fly very low as a result and is best avoided.
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