The convertible collar offers a bit more structure to a shirt or blouse than the Peter Pan, but is not as tailored as a banded shirt collar. It can be worn open or closed (hence the name), and looks very nice in a contrast fabric.
Worn open, the collar creates a lapel for a tailored, almost jacket-like opening. It sits open much more elegantly than a traditional, tailored shirt collar, yet does not appear too casual.
It is, however, the collar used on camp shirts, often made of brightly colored prints (Hawaiian shirts use the convertible collar, for example).
Worn closed, it make look a bit like a Peter Pan. But unlike the Peter Pan, this collar rolls at the neck and does not lay flat against the body of the blouse.
Start by tracing your blouse neckline and placing a mark 1" from the center front. This is where the collar will end; it does not overlap at center front. This will also help to create the lapel when worn open.
Then measure the front neck from that point to the shoulder (record the measurement), and measure the back neck from center back to the shoulder (record that measurement, too).
If you mirror this draft (or trace it on folded paper, with the fold along AC, the center back line), it will look like the pattern above. Be sure to mark your shoulders and center back; it's much easier to sew with markings.
This collar, you'll notice, does not follow the shape of the neckline and is, in fact, almost straight. That is what allows the collar to fold nicely and stand up near the neck, rather than laying flat against the blouse.
There are several more steps to making this pattern than a simple Peter Pan, but it's not hard, and it looks so nice!