HO or H0 is a railway modelling scale whi9ch uses a scale of 1:87 (3.5 mm to 1 foot). The rails are spaced 16.5 mm (0.650 in) apart for modelling 1,435 mm (4 ft. 8 1⁄2 in) standard gauge tracks and trains in HO. The HO scale is the most popular scale in railway modeling worldwide. The name HO comes from 1:87 scale being half that of O scale, which used to be the smallest of the series of older and larger 0, 1, 2 and 3 gauges which were introduced around 1900 by Märklin. In English speaking markets, it is written as HO, but in other markets, the letters H and number 0 (zero) are used.
The term HO can be stretched in model railroading. Some manufacturer in the UK have marketed railway items such as figures and detail items as “HO/OO” so as to appeal to modelers in both scales. The actual scale sometimes is OO, and the difference is negligible (about 1:82). These items may therefore be marketed as HO, especially in the US. Also, some producers tend to label any small scale model as HO scale regardless of the scale in order to improve their sales to hobbyists and modelers. For example, there may be a great variation in the sizes of HO automobiles from different manufacturers.
The “gauge” of a rail system is the distance between the inside edges of the railheads. It is distinct from the concept of “scale”, though the terms are often used interchangeably in rail modelling. “Scale” describes the size of a modeled object relative to its prototype. Prototype rail systems use a variety of track gauges, so several different gauges can be modeled at the same scale.
The gauges used in HO scale are a selection of standard and narrow gauges. The standards for these gauges are defined by the NMRA (in North America) and the NEM (in Continental Europe). While the standards are in practice interchangeable, there are minor differences
Due to the huge popularity of the HO scale, a wide array of models, supplies and kits are produced. The annual HO scale catalog by Wm. K. Walthers, North America’s largest model railroad supplier, lists more than 1,000 pages of products in that scale alone. Models are generally available in three varieties:
- Ready-to-run models are fully ready for use right out of the box. Generally, this means couplers, trucks (bogies), and other integral parts are installed at the factory, although some super detailing parts may still need to be attached.
- Shake-the-box kits are simple, easy-to-assemble kits; a freight car might include a one-piece body, a chassis, trucks, couplers, and a weight, while a structure kit might include walls, windows, doors, and glazing. The name derives from the joke that no skill was required – shake the box, and the kit falls together. A common synonym is screwdriver kit as many can be assembled with a screwdriver and tweezers.
- Craftsman kits require a much higher level of skill to assemble and can include several hundred parts.
Several manufacturers also market individual supplies in addition to these kits for super detailing, kitbashing and scratch building.