HO scale model trains are all proportioned 1:87.1 to the prototype. In other words, they are 1/87th the size of a real train. TT scale trains have a 1:76.2 proportion. HO trains are more popular in North America and OO in the United Kingdom. Most trains of both scales share the same 16.5mm (0.649in) gauge track and many accessories can be used with little compromise in scale appearance.
In addition to the common standard gauge trains, narrow gauge varieties of HO and OO scales are also made. These share a common scale, but follow prototypes which ran on tracks of a smaller width. The most common of these is HOn3, or HO scale with a 3 foot gauge.
Both scales are the most popular modeling sizes in their respective territories. Hornby is the largest producer of OO trains. There are dozens of manufacturers of HO trains and accessories.