Here are some instructions for drafting a convertible collar--I made this one to complete the pattern for my Friday Frock, a shirt-dress from Bernina that's missing the collar piece.
Just a quick reminder, for anyone who isn't a regular reader of this blog. I use Norma Hollen's book Pattern Making by the Flat Pattern Method as a reference for all of my drafting, and I want to credit her appropriately. If you're interested in drafting, her book is an excellent resource.
Here's the definition of a "convertible collar": Unlike a regular, or "full-roll" collar, a convertible collar is designed to lie nicely if the collar is left open, and not buttoned all the way to the top. To accommodate this, the neckline edge of the convertible collar is angled slightly, so it matches the curve of the shirt's neckline edge. Since women's collared shirts are rarely buttoned to the top, when we think about a "shirt collar," what we usually have in mind is a convertible collar.
To begin, we'll draft a full-roll collar, which is ridiculously easy. Then we'll modify it to create the convertible collar.
Then, draw a rectangle that is the length of your neckline edge measurement (here 7.5 inches) and 3 inches wide. Measure the distance between the Center Back and your shoulder seam, and mark it on your collar. To create the point of your collar, extend one of the long edges of that rectangle by about an inch (more or less, depending on the pointiness of your desired collar), and draw a diagonal line connecting it to the corner of the shorter edge. That's it! That's your full-roll collar. In this picture, the lower (shorter) edge is the neckline edge, where you attach the collar to the shirt, and the left side is the center back, cut on the fold.
Now, to make the convertible collar, we want to create a slight angle in the neckline edge of our collar. To do this, measure 3/4 inches up from the lower right-hand corner, along the diagonal edge of the collar. Then draw a line from this point to your shoulder seam mark. The lower edge of your collar should now be angled slightly upward. As a consequence, it will also be slightly longer, so you'll need to adjust it. To do so, measure your new, angled neckline edge, and mark your desired length (in this case, remember, it was 7.5 inches). Shorten your collar by re-drawing the diagonal edge, beginning from the point you just marked and running parallel to the original line.
So, here's what your finished convertible collar should look like:
Of course, you still need to add seam allowances. And remember, the collar piece we've drafted gets cut on the fold, and you need to cut two of them: an upper collar and an under-collar.