The tension square is the instruction everybody skips and everybody regrets
Why it matters
Half a stitch per ten centimetres over a hundred stitches is five centimetres of width. That is the difference between fitting and not.
How to do it
Cast on about a third more stitches than the stated tension, knit ten centimetres, and measure in the middle rather than at the edges.
If it is wrong
Change needle size, not tension. Trying to knit tighter or looser than naturally does not hold over forty hours of work.
