So here's the low-down: Before you get your panties all up in a bunch, just know that that diamond insert is NOT a defect nor is it there just for show (Haha! That's right! Keep those legs close ladies and gents!)
That geometrical piece of fabric is called a diamond gusset. It helps to reinforce the seams at the crotch because the way our bodies move and how we position ourselves create a lot of tension in that area. That's tension in the seams, people. Get your mind out of the gutter please :)
The diamond gusset makes the crotch bigger (Alright, you don't need to go there. Let's keep it professional.) but not deeper so that it impedes movement. The result is a slack crotch in the pant which allows plenty of room of your legs to open up in. For example, you will find that it is common for motorcycle riders with jeans that have diamond gussets in them because of the way they have to sit on their motorcycles. If they didn't have the diamond insert, they may find the jeans intensely uncomfortable since there will be a lot of horizontal pulling right in that area.
Pants that you may find diamond gussets in are: rockclimbing pants, dancing tights, martial art uniform pants, horseback riding pants and basically others that are made specifically for you to move in comfort when you're doing stuff that requires you to stretch out your legs and groins.
Custom tailors also make use of this diamond gusset feature to help make dress pants comfortable for their clients. Men who carry a little more weight and have a wider seat area or thicker thighs will appreciate this feature.
I have had hundreds of customers who come into our store to have dress pants or jeans repaired in the crotch. That is one area where rips and tears are prone to happen. Unfortunately, most denim jeans or dress pants that you buy at retail stores don't have the diamond gusset feature. Overtime, all that stretching and thigh-rubbing will put holes into your beloved pants.
That's when you'll want to bring in your pants for us to fix ;) See below:
Both sides of the inner thighs were torn badly due to continuous pulling and rubbing but they're now nicely patched up!
