A slit in a dress, blouse, or skirt.
In order to fit a shirt over a head there needs to be some extra give to the neckline. Sometimes this happens because the material has a stretch to it - like with T shirts. Sometimes, with dresses in particular, there's a zipper. Recently I've learned how to make a placket that looks really sharp with a collar. It's like having a button down shirt except the buttons don't go all the way down.
First you need a pattern for your placket. You might be using a store bought pattern or you can figure out the size of the pieces you want. First figure out how long you want the placket to be we're going to call this number Y. You want to make sure you've left enough room for several buttons. You want to cut one rectangle 3" x Y.
The next shape is a little bizarre you can see what it looks like in the following picture. Draw this on paper before cutting into the fabric. Draw a rectangle 3" x (Y+1"). Cut it out and lay it facing you with the longer edges on the left and right and the shorter edges on the top and bottom. Draw a line 1/2" in from the left edge. Mark a point 1" up from the bottom along this line. Mark a point along the bottom 1" in from the right edge. Connect these two points. Mark a point 3/4" up from the bottom along the right edge of the rectangle. Connect this point to the one on the bottom. This will form an arrow, a little lopsided but we're taking some seam allowances into account. Draw a line from the left edge to the first line 1.5" up from the bottom.
This is what it will look like (this photo includes the interfacing which we will do in the next step) :
You need two pieces of interfacing. They're both going to be 1" x (Y-1") Iron them on 1/2" away from the left edge on the weird shaped one and 1/2" away from the right edge on the rectangle.
Line them up along the the right side of the front center as in the photo.
Sew the placket pieces to the front of the dress 1/2" away from the center. Stopping a half inch from the bottom of the rectangle and a half inch from the bottom edge of the other side.
Fold the seam allowance away from the center. and mark a dot a half inch up from the bottom of the rectangle, another dot horizontal and another dot 1/2" up from those two dots along the center. Like this:
Cut the front of the dress along the center line to the dot along the center line then cut from the center dot to each of the side dots. Forming an upsidedown Y.
Fold and iron the seam allowances like you see in the photo.
Now we're going to work on the side with the rectangle attached. So fold down the other side so it's out of the way.
Fold the rectangle in half with the right sides together, interfacing out. Sew just above the interfacing to the seam - which is 1/2" then clip the seam allowances like this:
Turn it right side out and iron
Do the same thing on the other side. This will form the point. You will probably need to fiddle with it to get it nice looking.
Edgesew (sew close to the edge) around the rectangle one. Draw a line an inch and a half up from the bottom point. Edgesew around the sides and top of the pointed one starting and stopping just below the line. Layer the pieces bottom layer, the little triangle flap of the front of the dress, then the top layer. Sew these layers together by sewing across the line you drew and then edgsew around the bottom portion of the placket.
Add button holes and buttons if you're using them. Finish the neck line with whatever collar you want to finish it with.