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Glen Duff (from Manx: black or dark glen) is a point on the Mountain Course used for the Isle of Man TT and Manx Grand Prix races situated between the 21st and 22nd Milestone road-side markers on the 37+ mile circuitous road-race course, measured from the startline at the TT Grandstand,
It is situated on the A3 Castletown to Ramsey road and the side junction with the B14 Road of the Ayre (Bernaharra Road) in the parish of Lezayre in the Isle of Man.
Glen Duff was part of the Four Inch Course used for the Tourist Trophy car races held between 1908 and 1922, and part of the Snaefell Mountain Course used since 1911 for the TT and from 1923 for the Manx Grand Prix races.
The Engineering Work Section, Highways Division of the Department of Transport has a depot at Glen Duff, including a storage facility known as the igloo, for road salt treatments during the winter.