Single wire for interchangeable circular needles. Wire is available in three sizes:
M1.8 fits needle size 2.5mm - 3.25mm
M2 fits needle size 3.5mm - 5mm
M4 fits needle size 5.5mm - 10mm

Wires are available in four lengths, the length is measured from needle tip to the end of the needle.

25 cm = 40 cm (only fits 10 cm needle length)

32 cm = 60 cm

52 cm = 80 cm

72 cm = 100 cm