Sock sizes: why 39–42 is not one size
The same sock on a 39 and on a 42 fits differently: tight across the instep here, folded at the heel there. How to choose when you sit between sizes — and why a «universal» size is always a compromise.
Socks are sold in ranges: 39–42, 43–46. A range of three sizes means every pair must either stretch or fold — the only question is where.
What happens when the size is wrong
Too small: the cuff presses, by evening there is a mark on the calf, the heel drifts forward and the toe presses against the toes. The sock does not tear — it is simply under tension all the time, and the wool fibre wears out faster.
Too large: a fold forms under the heel. It is that fold, not the sole, that rubs the sock from the inside — the hole appears in the fourth or fifth week.
How to choose between sizes
If your foot is 41.5, take the smaller range for a close fit and narrow shoes; the larger one if your feet swell by evening or you have a high instep.
The test when trying on: the sock should sit without tension across the foot, and the cuff should leave no pressed mark after ten minutes.
What gives room besides the size
Two to three per cent elastane gives a pair roughly a size and a half of reserve. A cuff with elastane holds for years; one knitted on plain thread stretches within a month.
Density of the knit matters more than stretch: a dense pair holds its shape and follows the foot, a loose one widens and shrinks in length.
The rule: the size of a sock is set not by the number on the pack, but by where the pair has to stretch.