How does a man with a 38 waist comfortably get into a 36 pant?

If you've been in the business a while, you've likely seen this scenario before. You put a tape measure around a man's waist and it measures 38, and the customer immediately responds "38? . . . I always wear a 36". At first you don't believe him because you saw with your own eyes that the tape said 38, and you may even measure again to be sure. He still measures 38 the second time, so you put him in the dressing room with sizes 36 AND 38 to be safe. He comes out wearing the 36 and it fits. The reason is that this customer has a very FLAT seat.

When sizing a man for a pair of pants, their seat measurement will govern what size he needs more than his waist measurement. If he has a large seat, it's best to sell him a larger size than his waist measurement, since there isn't a lot of material to let out in the seat of a pant, while taking in the waist is a simple alteration. It's better to have the seat fit well first, and then deal with your waist alteration.

If he has a small seat compared to his waist, he will often be able to go down one or two sizes from his waist measurement, since pants are designed to allow for the curve of the seat immediately below the waistband. That's why pants have darts (seams) in the back from the waistband to the pockets. The darts allow for the seat area of the pant to be larger than the waist, while tapering quickly up to the waistband in the small distance between the back pockets and the waistband. If a person has very little curve to his seat, he doesn't need the sharp curve that's built into the pant right below the waistband, so he can "sneak" into the smaller size.

In the diagram to the left, the dotted line shows how a person with a flat seat doesn't need the curve found in most pants, explaining why he can get into a smaller size.