Surplice necklines cross over the chest: two mirrored tangents to a conventional neckline that reach from the sides of the neck towards the other side of the body. Sometimes one or both of the diagonals are cut short after they cross and are joined to a seam without reaching fully to the side, often they'll reach fully across and join the side side at the waist.
If you have a substantial chest, beware: I'm told that surplice necklines were briefly very popular in home-made '70s leotards, until folks found they tended to collapse and reveal all. You may find a strategic hook-and-eye or button sufficient to stave off disaster.
I don't have a substantial chest, and the surplice is one of my two favourite necklines because I prefer ones that only involve fabric - no zips or other fastenings. That said, the seam across the chest is irksome so my main favourite is a hoodless version of the Origami Zentai: it doesn't have a chest seam, but it's a little harder to take off.
Surplice
Whereas a normal snug neckline is only the circumference of the neck so can only stretch a little, this (single) surplice pattern can stretch enough to admit a suitably proportioned person. Here the neckline reaches just below the underarm, and the doubled layer of chest fabric is joined to the single layer that makes rest of the front (pale blue in the diagram), but other arrangements are possible. The two parts of this surplice layer two mirror-image pieces as shown.
A casing is sewn along the neckline, and the casing should hold a length
of elastic that is an inch shorter, the ends of the casing and the elastic
should be caught into the body's side seams. (A casing is a long rectangle
that ends up wrong sides together, sewn along its free ends. You can thread
elastic through it.)
I'm still trying to work out how to calculate the minimum practical length
for surplice necklines. The longest they have to stretch is when they're
being slipped over the elbow, which seems to involve some longitudinal
stretching (actually shear stretching) in the doubled chest panels, so simply
making sure the neckline casing is long enough before sewing it to the
neckline isn't sufficient: I think about a half of the neckline is restricted
to the longitudinal stretch factor.
Length of neckline
Drafting the surplice
I've always drafted the surplice at the same time and in the same way as I draft the raglan sleeves, so the neckline has a similar bend to the sleeve seams. Having tried a simple diagonal, I would recommend the bent neckline because it helps keep the elasticated casing away from the base of the throat (increasing comfort), and it also makes the casing marginally longer so the garment is easier to don or doff.
Notice that the surplice line in the raglan diagram is dropped into the pale rectangle, so that the casing is caught into the side seam under the arm. The top of the casing should be below the sleeve (an inch or so), or the casing will dig into your underarm.
Notice the dark blue rectangle - this adds room to prevent the neckline/casing seam from making it hard to sew the body/panel seam.
You will need to add seam allowance to the surplice panels because the dimensions are small enough to make that significant.
Sewing
The elastic isn't mandatory, but I recommend it to prevent the surplice becoming floppy across the chest.
- Complete the edge to which each neckline casing is to be sewn, but no
more:
- For raglan sleeves, sew the sleeves to the body, but don't sew the long sleeve seam.
- For in-set sleeves, sew the shoulder-top seams, but don't sew the arms in.
- Cut a rectangle as long as your neckline and almost twice as wide as your elastic (plus seam allowance). Fold it wrong sides together and sew its long edges together to make a casing. (Make sure before you cut that the velvet knap runs in the short direction.)
- Double-check the sewn casing can stretch from your underarm, around your neck, down your back to your elbow, and then across your chest to your other underarm. Check the same with the elastic you intend to cut, below.
- (If the casing is not long enough, unpick the shoulder darts and sew in identical long triangles whose base at the neckline will extend the neckline enough so a new casing will stretch enough. Tip: make the first casing too long and cut it to size, instead of finding it's too short and having to make another.)
- Sew the casing to the neckline.
- Sew the bottom edge of the two surplice panels together.
- Sew the bottom edge of the two surplice panels to the body.
- Cut a length of elastic, one inch shorter than the casing.
- Thread the elastic through the casing and secure the ends.
- While sewing the side bodyseams, catch the ends of the casing and the elastic.
Body with side seams
For a body style with side seams (eg four-panel, gusset leg, half in-set), the doubled upper chest panels are simply sewn to the front, and the whole is then treated like a standard front (except that the neckline's casing should be sewn on before the side seams are sewn, so its ends can be incorporated into the side seams).
Body without side seams
For a body style without side seams (eg one-panel, two-panel), you should introduce short side seams extending from the armpit, down a couple of inches, and particularly one inch below where the doubled panel is to be sewn to the body, so the horizontal seam is easy to create, and can be sewn into the introduced side seams (which should also catch the neckline casing).
[I believe the ends of the surplice could be caught into the seam that attaches the doubled upper chest panels, the short introduced side seams could be omitted and the sleeve seam could run fully across the chest and along the other arm, but I haven't tried that yet.]
Double surplice
The double surplice will work for most body types and allows a tighter circumferential scaling, because the neckline only has to stretch enough to let the arm in before it's hooked over the shoulder, but it suffers from a v-neck at the back which looks odd and can be drafty, and the underarm line is a seam all the way around.
Making a double surplice simply involves applying the single surplice method to the back of the garment as well as the front. Additional factors to consider:
- The fabric can end up several layers thick, with two layers of elastic, when neckline casings are taken into account.
- One could stagger the front and back casings higher and lower along the side seam to mitigate this.
- It's also a good idea to turn the serger by hand to avoid straining the motor.
- It helps with control to sew some of the surplice panel layers together along the undearm seam and side seam before sewing the whole lot.