One of the unique features of Wild Gears is the ability to make parallel lines in the designs. This is done by using the medium or large pen holes and then by putting a plastic doughnut piece in to that hole and repeating the design using the smaller hole.
The colours from small to large progress: clear, white, green, orange. The medium doughnut pieces come with three of each colour and go in 1 mm step sizes. The large doughnut pieces have twice as many steps so the colour pattern repeats twice and each set of doughnut pieces comes with one of each 8 sizes.
A card of doughnut pieces is available: here