Diagonal Seam Tape
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 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.
The red line in the center is aligned with the needle of your machine, and the black lines on either side are 1/4″ to each side from the center of the lines, to represent a 1/4″ seam allowance. It's particularly useful if you don't have a 1/4" foot, or do a lot of diagonal seams.