A cap sleeve is more of a decorative detail than a true sleeve - but it's a great-looking addition to what would otherwise be a simple arm-baring top.
Like its name implies, this sleeve consists of little more than the cap itself, and generally doesn't go all the way around the armhole.
As a shoulder detail, it adds a bit of interest and coverage when you want something just a bit more than sleeveless, but less than a typical short sleeve.
And it's easy to draft and to sew!
You'll start with your blouse sleeve block (the sketch shows only the upper part of the block, since we're making a very short item).
Start by shortening the cap itself by about 1/2". This reduces (or almost eliminates) the ease at the cap; it's not needed in this case.
Then mark a point on the grainline about 1 1/2" to 2" above the bicep line. From there, draw a gentle curve to each side of the cap, below the notches (the ends will be about 3/4" above the bicep line). This is your hemline.
The end result is a very short sleeve, which does not surround the arm. Be sure to mark where the sleeve ends on the front and back blouse pieces!
For a very casual top, you could turn and stitch the hem, but this sleeve looks best with a hem facing. The facing should be applied before setting the sleeve into the armhole.
And don't forget to finish your pattern with seam allowances, hem allowance, facings - or whatever is required of your style.
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