Why Would You Use A Curved Crotch Seam?

If you ever needed a reason to join our Facebook group, it's because of questions like the one Sarah asked yesterday. She wanted to know about curved crotch seams ... do they create a better fit or prevent camel toe/wedgies? She explained she'd come across other patterns, lingerie specifically, that use a curved crotch seam and wondered about the benefits are of doing it this way?


Well the curved crotch seam is actually a throw back from the days of low stretch fabrics (the original 100% nylon knits) from the 50's ... it was typically used in full brief undies for women that also had back quarter seams trying to create shape ... it's really designed to work with those. People forgot why it was done originally and just kept doing it. With modern stretch fabrics and much smaller width bottoms its probably of no real advantage anymore .... it can't really hurt (you need a seam anyway), but it's certain NOT going to prevent camel toe ... indeed it may make it more likely in some cases as your shortening the center line with out any account for that in the leg line.


Sarah provided some else's image so I've drafted one of mine in the same width as her example. The red line represents the common overly narrow back we're seeing on many reversible bikini bottoms in which "hungry butt" and camel toe are now very common place ... and it should be obvious as to why ... there's simply nothing left to hold onto the cheeks and the crotch width is so narrow the only thing anchoring the entire garment in place is ... well, the crotch itself, hence camel toe. Current PDF pattern designers of these styles think that tightening up the sidebands will hold the leg line out onto the cheek more ... .this demonstrates a lack of understanding of the fabric ... when you stretch in one direction, you increase tension in the perpendicular as well (basic first grade knit theory for those who went to fashion school) ... so what happens is they just effectively shortened the center back line further, turning it into even more of a thong with greater camel toe. So because they don't know how to fix it they've called it a new style .... yes, high waisted, wide band bottoms with hungry butt are a style.


But some people don't like that scrunched up look so they modified things again by putting in a curved crotch seam to try to contour into the bottom more gently. Umm guys all you did was shorten the center back even more???? If you want to fix this problem and contour properly then you need a CB seam (the light blue line) ... it's an even shorter CB length but now it sits properly in the butt and contours out over whatever width leg line you designed ... furthermore you can now reduce the tension on your side seams and eliminate camel toe due to excessive mid line tension.


But this isn't the only cause of Camel Toe. Camel toe is mostly due to pelvic tilt which requires repositioning the crotch section as a whole. In the garment Sarah provided as an example it wouldn't make any difference at all because the rear leg line coming from the crotch seam is around 90 degrees to the seam ... it tends to make a difference for leg lines over 120 degrees or more from the seam ... 90 or less will just shorten the CB line causing 45 degree ripples at the lower line pointing to the enter of the seam. There's an entire section on pelvic tilt and gusset mapping in the group. Another reason for camel toe is the nature of reversible swimwear with two layers of top fabric. People say it's thicker so it reduces the effect. Well yes and no ... this has been tested for years with multiple layers of lining well before reversible swimwear. The more layers of fabric the less likely the fabric will stretch in the center of the panel, instead flattening body features like the bust. But in the crotch region when the middle of the panel is resistant to stretch and the leg line stretches due to elastic only cms away, the section over the crotch creases in the center (convex curve with perpendicular tension) and so a crease drops right down the center line exaggerating camel toe more than usual (even though everything is still being flattened) .... it's about vertical fabric tension varying greatly across a small area with little to no horizontal tension. Why? ... because it's too narrow! Why is it too narrow? .... because everyone thinks they have to clear the leg muscles rather than bring the gusset forward. What I call cascading errors because of a lack of understanding of the principles because of a lack of formal education (sorry if I sound grumpy but there's no other way to say it) ... I don't expect home sewists to understand all this, but I do expect people selling patterns to understand it perfectly ... yet usually they have less understanding than the people actually doing the sewing!


Now bunching at the CB near the crotch is caused by a number of different things:

- cheeks centered to the side

- too narrow back panel at the seam going to too wide too quickly

- leg line apex too far back (certainly looked like that in Sarah's example)

- garment too tight at the waist

- anterior pelvic tilt

- two layers of top fabric (common in reversibles)


You really need to look at each garment and assess the design individually. I prefer to avoid the problem entirely by using a curved CB seam (light blue line) but reversible swimwear people seem opposed to using them because it's another sewing line, so they tend to look for other compromise ways around the problem.



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