Jo Blocks
From Kerbal Space Program Wiki
Jo blocks (Johansson gauges, gauge blocks) are, in KSP, a concept rather than a mod or set of physical parts. Real Jo blocks are small rectangular precision-machined parts that are stacked together or wrung to make up arbitrary lengths. Ingame, every instance of each part is precisely as long as every other instance; but it is not always immediately clear what that length is.