I. CROTCH - The distance between the normal waistline in front to the normal waistline in back.

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CROTCH
The distance between the normal waistline in the front to the normal waistline in the back.

The natural waistline in the front is normally in level with your navel (bellybutton). The natural waistline in the back is normally just above the level of your navel. The location of this measure can vary a little from person to person. Measure stretched measure tape and do not add anything.