HO scale is a model railway scale using a 1:87 scale, which is the most popular scale of model railway in the world. The rails are spaced 16.5 millimeters apart for modeling 1,435 mm standard gauge tracks and trains in HO. The HO scale is half the size of the O scale, making it large enough to accommodate a great deal of detail in finer models, more so than the smaller N and Z scales, and can also be easily handled by children. HO scale modelers tend to have the widest variety of models available for rolling stock, locomotives, and buildings. The minimum requirement for a HO scale turning radius is 15 inches, HO scale rails are 16.5 mm apart, and the average HO scale train is about 15 to 22 inches long.