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There are other sites that describe the TT/MGP course for their followers but this description is based around Marshalling the course.
The TT/MGP course is 37¾ miles long and for the purposes of Marshalling, is divided into 12 sectors.
Each sector is managed by a Chief Sector Marshal (CSM) and a number of Deputy Sector Marshals (DSM). Each DSM controls a section of the sector, usually a significant corner or a group of marshaling positions within a given area.
The total number of marshals needed to be in the correct positions around the course before a race or a practice can start is just over 500. Each sector has a 'Minimum manning level' that is calculated to provide minimal cover at the various spots around the course.