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 exactly 1/4″ to each side, to represent a 1/4″ seam allowance.
The tape removes cleanly, but it is recommend that you remove or replace the tape often or after each project to avoid any residue on your machine, especially in high temperatures.
Please see here for usage instructions.