Before there was geometric dimensioning and tolerancing (or GD&T for short), manufacturers relied almost entirely upon linear dimensioning to achieve standardization of parts.
Linear dimensioning is simply the practice of measuring a given distance to the most exact possible degree of accuracy. It was during the Second World War that GD&T was first discovered and applied. A British inspector of submarine torpedo parts, a man named Stanley Parker, discovered that certain components which wouldn’t normally seem qualified for military specifications were actually fit to be used in torpedoes. Instead of throwing the parts away as would normally be done, Parker realized that if the components were mated together with other parts to a reasonable degree of tolerance, then they still met the requirements of military specifications, and were good to go. Naturally, this saved Parker time and effort in the fast-paced business of providing for the British Royal Navy.