What is meant by US pitch, 2:1, 3:1, 4:1, and 5:1?
US pitch = 21 holes in total on A4 paper.
2:1 = Two holes punched per inch. 23 holes in total on A4 paper.
3:1 = Three holes punched per inch. 34 holes in total on A4 paper.
4:1 = Four holes punched per inch. 46/47 holes in total on A4 paper.
5:1 = Five holes punched per inch. 59 holes in total on A4 paper.