Some patterns require that you inset a piece between two other pieces that are already sewn together. The traditional 8-pointed star is a perfect example. This pattern is made with eight 60º diamonds. This tutorial will take you through the process step by step so that you will get perfectly flat results every time.
This is often called a Y seam because of the shape the three seams would make if you think of them as one unit. The red Y in the illustration below shows the path of the seams.
Thinking of them this way is actually the root of the problem. You need to approach each seam as an individual unit. Each seam must end without including the seam allowance. This allows you to pivot the pieces and attach them without creating a tuck.
It helps if you can cut out the pieces as shown in the illustration, with the points trimmed. This automatically decreases the amount of bulk you will have in the middle where the eight pieces come together. There are tools that give you this option and certain templates, like those from Marti Michel, come with the blunted edges. Templates created in Electric Quilt are drawn with the corners trimmed.
Once your pieces are cut out, lay them out next to your sewing machine, just as you will sew them together. As you sew each piece, replace it in the layout. It is important that you always know which unit you are working on.
The picture below shows how to mark the end of the seam allowance on the diamonds. Lightly draw a line 1/4" from the outside edge on all four sides. The end point of the seam and the beginning of the seam allowance occurs where the lines cross at each corner. Darken the X so you will be able to see it clearly while sewing. A chalk pencil can be used on the dark fabrics.
Use a clear ruler to mark seam allowances
Light diamond is folded back to show where stitching ends
For the 8-pointed star, step one is to sew all the star points together in pairs. One pair will be in each quarter of the star. Above, you see one closed pair at left and one open pair at right. The light blue diamond is folded back so you can see that it is NOT sewn all the way to the end. You stop when you get to the X you made to mark the seam allowance.
X marks the spot
At BOTH ends of each diamond, begin and end the sewing from the center of the X. This will leave 1/4" unsewn on each end of the seam.
Red arrows show the seam allowance and blue arrows show X marks
Red arrows on the picture at left show the seam allowance turned under. It is possible to turn under the seam allowance because the end is not sewn down. The un-sewn seam allowance lets us swing the adjacent piece out of the way, shown on the right, as the background is sewn to the star points.
All the seams on one side are pressed in one direction and the place where they meet swirls in a half circle
When all the pairs are sewn together, sew them together into two sets of four. One set is shown above. All the seams will be pressed in one direction as shown in the bottom photo.
Adding the background pieces
You can take time to mark the seam allowances on the background pieces or always sew with the star pieces on top, so you can see those marks. The square goes on first. Place the background piece on top of the star piece, having the outside ends even. This is another place where it helps to have the long points trimmed. If the tips were not blunted, a long point would stick down below the square background piece.
With the background piece against the sewing machine, place your needle in the exact hole where you stopped sewing the diamond together. Be sure that the other star point is out of the way. Sew to the outer edge of the star. Press to set seam.
Fold the background square right side up as shown, above left. Press. The pencil in the picture shows that the second side is not sewn down.
Fold the pieces over so that the unsewn side is positioned against the dark star point. Be sure that only the background and the dark star fabric will be caught in the stitching. The loose seam allowance will allow you to easily keep the other parts out of the way. Once again, begin stitching by putting the needle exactly in the center of the X, where the last stitch ended when you sewed the diamonds together, and sew to the outer edge.
Continue adding background pieces in the same way as you move across the outside of the star. You will end with the triangle on the far right. Notice that you did NOT start with the triangle on the left.
One half with four background pieces attached
The triangle pieces do not create a straight line across the top of the four sewn diamonds. Therefore, we will sew one triangle to set A and one triangle to the opposite site of set B, leaving us with two final set in seams to complete. One half is shown above.
Put the two units right sides together. Start in the center of the star, being sure that you are starting your stitching in the middle of the X. Stitch to the X at the end of the diamond. Stop and cut your thread. Fold the background triangle over to the adjacent diamond and sew from the X to the outer edge, the same way you inserted all the other background pieces.
Return to the middle and repeat the stitching order for the other side, stitching from X to X, cutting the thread, folding the background triangle over to the adjacent diamond and sewing the last seam to the outer edge.
Assembling the final two units takes four seams. The arrows show the direction of sewing.