Also known as the rise or the saddle. This measurement determines the crotch length, and if measured incorrectly can either give a "wedgie" or a very low crotch (making it difficult to widen the leg-stance).
To measure, begin at the front and place the tape measure at the top of the waistband (I-2). Pass the tape measure down between the legs, and up to the top of the back of the waistband. Let the measuring tape contact the body curves, without beeing too tight (wedgie) or too lose.
TIP: wear pants or shorts that are an example of how you want these to fit.