International shoe sizing system (ISO 9407) stating foot length directly in millimetres — the standard for ski boots and skates.
Mondopoint 27.5 simply means 275 millimetres of foot length. The system exists because EU and UK sizes vary by up to two numbers between manufacturers — in a ski boot that has to fit snugly, that's the difference between a good day and pain from the second run onward. How to measure: stand with a sock and your heel against a wall, mark the tip of your longest toe and measure the distance; measure both feet, take the larger, ideally in the evening when feet are slightly swollen. Note: ski boots also come in last widths (usually 98–104 mm) — length alone doesn't make a boot fit.