This is by far the most common scale in use. To sort out any confusion, HO models are built to 1:87th scale. This simply means all the dimensions of the full size item (the 'prototype') are divided by 87. So 87 models all lined up would be the same length as the prototype. 87 stacked up would equal the height of the prototype. To put it another way. HO is also known as 3.5mm scale. 3.5mm on the model represents 1 foot (12 inches) on the prototype or full size item. HO scale actually stands for Half O.
