Trueing the Crotch/Rise Intersection (Eliminating “Stabby Crotch”)
June 13, 2016
Sewing patterns must be “trued” to ensure a smooth edge wherever a seam ends, so that the joined pieces are ready for the next step in construction. The top and bottom corners of each pattern edge should be as close as possible to 90-degrees (a right angle) so that after stitching the seams together, the resulting edge will be a smooth straight line (180-degrees).
If the corners are LESS than 90-degrees, the seam-stitching will result in a little triangle poking out, which will make it difficult to go onto the next step, whether it’s hemming, or folding the fabric edge over to stitch an elastic waist casing, or simply matching up the following seam intersection. Previously I showed how this could be a problem on a dress bodice at a neckline casing, and this time I’ll show how it could be problematic in pants at a waistline casing and more importantly at the crotch seam intersection.
Lately I’ve seen a bunch of patterns with crotch intersections that haven’t been trued, which makes them difficult to stitch. An illustration of this problem is shown here:
It doesn’t matter which sequence-of-stitching you choose (rises then inseams as in activewear, or inseams then rises as in dress pants), there will be an extra little triangle of fabric that makes a “bump” preventing a smooth edge. It needs to be trimmed off. In factory production, the stitchers would get tired of serging it off (it is also a waste of fabric that makes the pattern marker less efficient), and the pattern would be sent back for trueing: squaring off the corners at the bottom of the rises.
I’ll compare 2 patterns to show the difference trueing makes at the crotch/rise intersection. Both are for kids’ stretch knit shorts, with a very similar design and a single pattern piece (front and back with no outseam). On the left is vintage Kwik Sew #3024 ; on the right is the brand new Simple Life “Elle”:
First up is Kwik Sew 3024: all seam corners are trued to 90-degrees. Here are the squared TOP corners of the rises (WAIST) : when the Center-Front rise and Center-Back rise are stitched, both will yield a smooth 180-degrees along the top edge, ready for folding over to create the waist casing:
Here are the squared BOTTOM corners of the rises (CROTCH): when the Center-Front rise and Center-Back rise are stitched, both will yield a smooth 180-degrees, ready for joining Front to Back easily when stitching the inseam:
Now look at the “Elle” shorts. The Center-Front TOP corners (WAIST) ( are obtuse (over 90-degrees) and when the rise is stitched, the CF will have a valley:
The Center-Back TOP corners are acute (under 90-degrees) and when the rise is stitched, the CB will have a peak:
That’s going to make it difficult to fold over the top edge to create the waist casing: the Center Back will have a peak that needs to be squished in, and the Center-Front will have a valley that needs to be stretched:
The fix is easy: Left photo, use a straight-edge to square off the pattern at Center-Back to create a 90-degree angle. Right photo: use a hip-curve to slightly scoop the top waist at Center-Front so that the center-front is 90-degrees (right):
Now the top edge has the same shape as the Kwik-Sew waist, with squared-off 90-degree corners, so no peaks and valleys when folding over the edge to create the elastic casing waist:
Here’s a close-up of the Center-Front Rise corner:
I was told the concept wasn’t clear, I hope this helps (black line is where pattern was trued….the squared corner need only go a couple of inches, then the line fades into the original waist edge):
But the bigger problem is the “Stabby Crotch”: at the CROTCH intersection, the BOTTOM corners of the rises are not trued; they have severely acute angles:
I imagine the reason for this was to create a slim leg by putting all of the tapering at the inseam (since there is no outseam). However there is only so much leg width that you can remove from the inseam without causing a problem: when the inseams are stitched, the result will be a peak similar to the illustration example at the beginning of this post:
(Note, if the sequence-of-stitching is reversed, rises first then inseams, the same peak occurs).
I sent a message to the designer last week asking how to deal with this, but haven’t heard back.
Let’s say that you stitch according to the shape of the pattern using the standard seam allowance, and use the sequence-of-stitching in the instructions (rises then inseams). Here are your cut pattern pieces (2 back, 2 front) and what they look like after stitching the rises (To protect the designer’s work, I’m only showing small parts of the pattern pieces to illustrate a process):
Putting right sides together and stitching the inseam, the result looks like this:
Unfold the stitched inseam and you have a 3-dimensional peak, either sticking out from the crotch area, or inwards (either way, awkwardly uncomfortable):
See? “Stabby Crotch”. So you’d probably intuitively re-stitch to get rid of the peak, or serge off the triangle:
And now you actually have trued your muslin, you’ve created all four90-degree corners.
Let’s go back to the pattern to see what has happened. You stitched across and got rid of the peak:
Now if you virtually “unstitch”, you can see what has happened to the pattern. The crotch rise intersection has been automatically trued:
However now there’s a new issue: the crotch width has been shortened. This may cause “crotch bite” in front and “wedgies” in back:
If you want to keep the original Crotch Width for a comfortable fit , you need to go back and true the original pattern. First thing is to square off the BOTTOM corner of the rises (= TOP corners of the inseams):
Next use a French curve to shape the inseam and square off the BOTTOM inseam corners where the inseam meets the hem. That’s going to add a lot to the leg opening, so I’m going to compromise** and slice off just the tips that would get cut off in production:
Now all of the corners are trued (sometimes I use Post-Its to note that I’ve squared off the corners) , and the new inseam angle is almost identical to that of the Kwik Sew pattern above :
Final step: pin inseams together and check for smooth crotch curve (only the largest size is corrected; each size needs to be trued separately for accuracy):
**You’ll notice that reducing the angle of the inseam (while trueing the crotch intersection) causes the leg openings to have a wider sweep at the hem than originally. However, having too sharp of an inseam angle, while ensuring a tapered fit pant, can also create problems: stress pull-lines where fabric wants to shift towards the inner thigh, creating “smile lines” at the front crotch and prevent the shorts from hanging smoothly, as you can see by the grainlines in these stripes (different pattern, same inseam-angle issue):
Pants fitting is always a balancing act between many coordinating (and sometimes competing) forces, which is why muslin-fitting is especially important with bottoms. Personally I feel that, especially for kids, comfort beats “style”.
That said, especially if your child has thin thighs, that you can remove some fullness from the out-seam area (there is no actual stitched outseam; you would pinch out a vertical pleat). You can also add an outseam by cutting the pattern vertically where an outseam would be, then starting at the hipline you can taper the leg as much as you want to, then be sure to add seam allowances.
I promise to move on from crotches….somehow a whole lot of crotch pattern “issues” have been on my radar lately….I have a backlog of dresses to investigate!