Since I started sewing lingerie, I have researched bra and undie grading.
I have graded these types of garments on my own and have found that there is a very simple and accurate way to grade an undie pattern.
First, what is grading? Grading is the process of creating a size range of patterns for a particular garment. It’s the process of creating a size 4, 2, and 0 pattern from a size 6 pattern or a size 8, 10, and 12 pattern from a size 6 pattern. Typically, a middle size such as a size 6 or a size medium is drafted and then graded up and down because grading can only be done two or three sizes at a time before pattern shapes are distorted.
To make it more confusing, grade rules depends on the particular garment. An example is the body length grade of a t-shirt. On this type of garment, the grade for body length depends on the length of the garment. If the body length is longer than 25,” then body length grades ½”, if the body length is between than 25” and 18,” then body length grades 3/8”, and if the body length is less than 18,” then body length grades ¼”. So essentially, every garment has a different grade and grading is done on a case-by-case basis.
I didn’t develop my own set of grading rules for undies. Instead, I used standard grading rules to determine the grade rule for a simple undie like the one shown above. When I worked in tech, we would “release” the grade of a particular garment once it was “approved for production.” At this stage, we would evaluate the garment and use standard grading rules to come up with a grade tailored to that style. Using the example of the t-shirt, if the body length was more than 25″, then the body length graded 1/2.” But what about the front neck drop? If the front neck drop was less than 5,” then it graded 1/4″ but if the front neck drop was more than 5,” then it graded 3/8.” So again, every garment had a different grade and grading is done on a case-by-case basis. This is exactly how I determined the grade for this type of undie.
Just like with everything else in this hobby, there are several ways to grade. I chose this method for two reasons. One, because of the simplicity of the an undie pattern and two, because it is based on the old school “shift” method. During my tech days, an older tech brought in a hand cranked grading machine she used way back when. By winding / cranking a handle on the side of the machine, just like a jack-in-the-box, the machine would shift the pattern up, out, and down and draw the new pattern.
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The first step in grading is defining a size range, such as XS-XL or 0-14, and making sure that your size range is consistent and proportional. This means that if a size 2 has a bust/waist/hip ratio of 33”/25”/35”, then a size 4 has a bust/waist/hip ratio of 34”/26”/36” – each one increases by 1.” A grade would not be proportional if a size 2 had a bust/waist/hip ratio of 33”/25”/35” and size 4 has a bust/waist/hip ratio of 35”/26”/36” – the bust increases by 2” while the waist and hips increase by 1.”
Now let’s get started. For most of my undies, I define the size range to be XS-XL (most stretch garments are alpha sized while most tailored/woven garments are numerically sized). Next, I define my grade rules. From size XS to sizes S and M, the total circumference from size to size at the waist and hip increases by 2” (which means that the garment increases 1” flat and each pattern piece increases ½” at waist and hip). From sizes M to L and XL, the total circumference from size to size at the waist and hip increases by 3” (which means the garment increases by 1 ½” flat and each pattern piece increases ¾” at waist and hip). Why does the grade rule “jump” from 1” to 1 ½” from size M to sizes L and XL? To be honest, I don’t know; it was the way I was taught in the industry.
Some textbooks say to grade the height of an undie depending on where the crotch seam is (if the crotch seam is located more towards the back of the undie, then the body length grade for the front is more and the body length grade for the back is less). But for simplicity, I keep the body length grade for all sizes the same for front and back – ½”. I’ve never had a problem doing it this way. Plus, undies are made with super stretchy fabrics so it will stretch to fit many sizes and shapes.
Why does the body width grade “jump” from M to L and XL but the body length grade stays the same for all sizes? This is going to sound odd to write and read, but it’s because as the body gets bigger, it gets wider at a greater rate than it gets taller.
The crotch width does not grade because from size to size, this area of the body does not get bigger/wider.
With grade rules defined, it’s now time to actually grade. The steps below are for the front pattern but the same applies to the back pattern.
On a large piece of paper, draw a vertical line the length of CF plus 5 inches
On a separate piece of paper, trace and cut out pattern. Make sure there are no seam allowances included (grading is always done without seam allowances)
Place the cut-out pattern on the larger piece of paper and lining up CF of pattern with vertical line, trace pattern
Remove cut-out pattern and draw a line perpendicular to vertical line at the crotch level (this is where the grade is “held”)
Mark ¼” up from waist at CF/SS and mark down ¼” from crotch at CF and side (this will make a total length grade of ½”)
Mark ½” out from side seam at top waist and leg opening
Using the cut-out pattern as a guide, draw in the new pattern shape. Shift up, shift out, shift down, and draw in new shape, keeping pattern at the hold line for each size increase. Because the pattern does not grade at the crotch, redraw leg opening by blending to original crotch width and ensuring the integrity of the leg opening curve is maintained (see diagram for clarity/more understanding).