4 mm scale is the most popular model railway scale used in the United Kingdom. The term refers to the use of 4 millimeters on the model equating to a distance of 1 foot (305 mm) on the prototype (1:76.2). It is also used for military modelling. For historical reasons, a number of different standards are employed. Standard gauge Three different gauges are used for modelling standard gauge railways in the United Kingdom. OO gauge OO gauge uses 4 mm scale with 16.5 mm gauge track, which is inaccurately narrow since it is correct for HO scale (1:87.1). It is the most popular standard in the UK for 4 mm scale trains and is produced by the two main manufacturers in the UK. The traditional standard for wheels and track is a very coarse one with extremely oversize rails and flanges; in recent years, some manufacturers have switched to using the American National Model Railroad Association HO standard S-4 instead. EM gauge EM was originally defined to use 18mm as the track gauge (hence the name: Eighteen Millimetres). This was revised, and today EM gauge uses an track gauge, which is closer to accurate but not fully to scale. It was developed as an early improvement on the standard OO system. It is still popular with FineScale modellers today because it utilises larger than scale flanges on the wheels of rolling stock, and because point and crossing (P&C) work is a little easier due to an overscale flangeway clearance. These two concessions in scaling down the prototype make for reliable running. This factor explains the continued popularity of EM as a scale for finescale modellers working today. P4 P4, otherwise known as Protofour, or "18.83", is a close to exact 4 mm:1 ft replica of real-life track and wheel dimensions. The gauge of track built to these standards is 18.83 mm. Along with EM Gauge It is also a popular choice for finescale modellers working today. The main advantage of P4 over the other 4 mm standards is that the wheel flanges are approximately to scale and the flangeway gaps on P&C work are also close to scale. This is most noticeable in photographs of the model where in many cases is it almost impossible to detect that you are looking at a model, as opposed to the real thing. The disadvantages of near to scale flanges and flangeway crossings