HO or H0 is the most popular scale

Scale model

A scale model is a physical model, a representation or copy of an object that is larger or smaller than the actual size of the object, which seeks to maintain the relative proportions of the physical size of the original object. Very often the scale model is used as a guide to making the object in...

 of model railway in the world.

According to the NMRA standard S-1.2 predominantly used in North America, in HO scale, 3.5 mm (0.137795275590551 in) represents 1 real foot (304.8 mm); this ratio works out to about 1:87.1. According to the MOROP

MOROP

MOROP is a European association that federates national associations of railroad and model railroad enthusiasts. MOROP was founded in Genoa in 1954 and now has its headquarters in Bern . In 2006, MOROP federated 22 national associations from 17 European countries, with a total of more than 30,000...

 standard NEM 010 predominantly used in Europe, the scale is exactly 1:87. In HO, rails are usually spaced 16.5 mm (0.649606299212598 in) apart which models the standard railroad gauge

Standard gauge

The standard gauge is a widely-used track gauge . Approximately 60% of the world's existing railway lines are built to this gauge...

 of .
The name HO is derived from the fact that its 1:87 scale is approximately half that of O scale

O scale

O scale is a scale commonly used for toy trains and model railroading. Originally introduced by German toy manufacturer Märklin around 1900, by the 1930s three-rail alternating current O gauge was the most common model railroad scale in the United States and remained so until the early 1960s...

 which was the smallest of the series of older and larger 0, 1, 2 and 3 scales introduced by Märklin

Märklin

Gebr. Märklin & Cie. GmbH or Märklin is a German toy company. The company was founded in 1859 and is based at Göppingen in Baden-Wurttemberg. Although it originally specialised in doll house accessories, today it is best known for model railways and technical toys...

 around 1900.