OO gauge

The terms OO gauge and OO scale (or more correctly but less commonly, 00 gauge and 00 scale) relate to the most popular standard gauge model railway standard in the United Kingdom, "00" is a variant of "H0", meaning Half-0, which historically derives (in increasing size order) from 0 scale, 1 scale and 2 scale, the most popular scales in the early 20th century. Since railway modellers invariably pronounce the zero as "oh" rather than "zero" (e.g. "double-oh" or "aitch-oh"), the scales are often written as OO, HO and O.

00 scale is one of several 4 mm-scale standards (4 mm to the foot or 1:76.2), and the only one to be marketed by major manufacturers of British-outline models.

Logically, to replicate the full-size ("prototype") standard gauge of 1435 mm (4 ft 8+1⁄2 in) the track gauge at 4 mm-to-the-foot scale would be 18.83 millimetres (0.741 inches). However, the gauge is 16.5 mm (0.65 in), which is the same as in H0 scale – 3.5 mm to the foot or 1:87. This oddity has historical origins: essentially, 00 scale involves 4 mm-to-the-foot bodies being mounted on 3.5 mm-to-the-foot track. The result is that 00 rolling stock appears to be running on narrow gauge. The anomaly led some 4 scale modellers in the 1960s to adopt a gauge of 18.2 mm (EM scale), soon followed by some who decided to adopt 18.83 mm and wheel/track proportions very close to full-scale practice (Protofour standards).