Diagonal Seam Tape by Cluck Cluck Sew
 
Diagonal Seam Tape is simply washi tape that is placed on the front plate of your sewing machine to act as a guide while you are sewing diagonal seams. The red line in the center is aligned with the needle of your machine, and the black lines on either side are exactly 1/4" to each side, to represent a 1/4" seam allowance. The tape is strong, thin, sticks well to almost any surface, and comes off easily. Each roll has 10 yards of tape so it should last a really long time.