A raglan sleeve is one where the sleeve reaches all the way to the neckline, covering the shoulder. A lot of sports and activewear uses raglan sleeves because it gives a looser fit, making movements easier, without being baggy. It's also easier to draft and sew than an in-set sleeve because the curves on both sides of the sleeve/torso seam match.
The raglan pattern is normally derived from the in-set pattern, but I haven't investigated that yet. I present how I've been drafting them, which is very closely related to how I've been drafting surplice necklines. (I've marked a surplice line in dark blue to give the general idea, but you can style this any way you like, perhaps by making it a curve that drops from H more vertically, or by drafting the entire surplice diagonal more vertical so its end is sewn into the underarm line.)
The shoulder/arm seam
The idea here is to make a curve that looks something like a straight line from the front, and which fits around the body and the arm at the same time.
- On your paper, draw a circle to represent the basic neckline. Plot a vertical line from its centre (C), extending out of the circle. This line will be the centre front and centre back of the body.
- Measure down the centre line from the bottom of the circle by the
underarm-clavicle length, and draw a perpendicular there. This gives the
underarm line for the front of the garment. Measure up from the top of the
circle by the underarm-nape measurement and plot a perpendicular to get the
underarm line for the back of the garment.
(An alternative step that I sometimes find easier: divide the scaled underarm-over-shoulder measurement in half and plot the circle with its centre r/2 below that half-way point. This puts exactly a third of the neck's circumference behind and two thirds in front. This is a different length to the underarm-clavicle/nape measures, and I haven't yet worked out which is better. I think this might just be tighter.)
- Plot horizontally along each underarm line for a quarter of the circumference of the body, on each side of the central front/back lines. We'll call this point U (or U' for the back).
- Along these underarm lines, also plot the 'flat width' (F) - where your chest's curvature is basically flat and the ruler wasn't more than about an inch from your chest [probably the distance between your nipples if you're a guy] - and the 'projected width' (P) which is the width appears to have if someone's looking at you straight-on.
- Plot a line from the centre of the circle (C) to the projected width point (P). It intersects the circle at H.
- Extend a vertical line up from the flat width mark (F).
- Find D, where CP crosses the flat width vertical (F). Plot DU.
- HDU is the chest's side of the chest/sleeve seam.
- Reflect HDU in HU to find E - either:
- fold the paper along HU and copy E from D, or
- plot the perpendicular of HU through D and double the distance D to HU along the perpendicular to reach E.
- Plot HEU, the arm's side of the chest/sleeve seam.
- If using a surplice neckline, add a rectangle (as shown at the bottom) to catch the surplice casing into. If using a double surplice, add one to the top too. Note that the surplice line doesn't necessarily extend from H, but is a tangent to the circle.
- Adding seam allowance to the neckline seams is vital.
- Add notches somewhere along the arm/sleeve curves - single notches on the front, double notches on the back. See the purply arrowheads on HD and HE, and the corresponding back seams.
- Plot a line for the sleeve centre line.
- Plot the arm circumferences across this line, half to each side.
The shoulder dart
- Take the shoulder, the yellow bit in the diagram above, and mark Z, the midpoint of UU', also the centre line of the arm piece. Cut the shoulder along the arm piece's centre line.
- Holding U and U' at the corresponding corners of the sleeve piece, move the Z corners away along the arm's centre line for the difference between the raised arm length and the dropped arm length.
- (The shoulder piece may be wider or narrower than the sleeve piece. The points Z on the shoulder pieces should still be moved away, and the dart should be cut from Z to the neckline points.)
- Tape the shoulder piece in place to the arm piece with extra paper to fill the gaps created.
- Fold the sleeve along the arm's centre line, and cut a smooth curve from and parallel to the cut edge of the shoulder piece to Z, parallel to the arm's centre fold (shown in red).
- Add seam allowance.
- Re-measure your neckline - front, back, and sleeve pieces - just to double-check it's long enough, remembering to subtract seam allowance from your calculations.
Taking account of the neckline and the body construction:
- Pin the arm front and shoulder front, right sides together, and sew.
- Pin the arm back and shoulder back, right sides together, and sew.
- (This is your last convenient opportunity to measure the neckline length and correct it if it's too short, by sewing in a long triangle to partially fill the Z dart sewn immediately below.)
- Pin the dart starting at Z, right sides together, and sew.
- Pin the arm sides (and body sides, if the arm sides continue into the body's side seam), right sides together, and sew.
If your chosen body pattern doesn't have seams along its sides, you have two choices:
- make the sleeves as a finished piece and sew along the shoulder/underarm seam like an in-set sleeve, or
- introduce some small seams near the underarms so you can sew the sleeve in as described. This latter combines well with surplice necklines.