Model railway scales are often identified by letters and numbers, such as Z, N, TT, OO and O. These designations have developed over many years and have different historical origins.

The scale ratio tells you how large the model is compared with the real railway. For example, a 1:76 scale model is 1/76th the size of its real-world counterpart.

The different designations can also be used alongside numbers to identify particular scale and gauge combinations. For example, TT:120 describes models built to 1:120 scale running on 12mm gauge track, while OO-9 describes 1:76 scale models running on 9mm gauge track to represent a narrow-gauge railway.

In simple terms: scale describes the size of the model, while gauge describes the distance between the rails.

The terms scale and gauge are sometimes used interchangeably, which can make choosing a model railway scale or gauge confusing. For example, OO has a scale of 4mm/ft and a gauge of 16.5mm, while HO has a scale of 3.5mm/ft and uses the same 16.5mm track gauge.

Put simply, scale refers to the size reduction from the real thing, whereas gauge is the measurement between the rails. Terms such as Z, N, OO and O refer to specific combinations of scale and gauge.

Understanding the distinction between scale and gauge becomes particularly important when working with narrow-gauge model railways. For example, an OO-9 model is built to the same 1:76 scale as OO, but uses a narrower 9mm gauge, representing a 2ft 3in prototype gauge.

The table below shows the principal model railway scales and gauge combinations currently used by modellers.

Rail code refers to the height of the rail, measured in thousandths of an inch. It should not be confused with track gauge, which is the distance between the inside faces of the rails.

PECO track is available in a range of rail codes across different scales and gauges. For example, N Gauge is available in Code 80 and Code 55, TT:120 uses Code 55, while OO/HO and O Gauge have a number of rail-code options. The choice of rail code can affect the appearance of the track and its suitability for different types of rolling stock.

These are some of the principal scales and gauge combinations supported by the model railway trade and widely used by modellers.

There are also some rarer and more unusual scale and gauge combinations. These tend to involve building much of the railway from scratch. S Scale (1:64), for example, is traditionally defined as 3/16in to the foot, while other modellers work in the all-metric 1:50 scale for metre-gauge railways.

Miniature railways such as the fifteen-inch-gauge Romney, Hythe and Dymchurch Railway can also be represented in 7mm scale, with trains running on N gauge track.

In the hands of a skilled and experienced modeller, these unusual scales can add variety and interest at exhibitions, although they may require more specialist modelling skills and research.

For railway modelling purposes, Standard Gauge is 4ft 8½in (1,435mm), referring to the distance between the rails. The gauge developed from dimensions used on earlier wagonways and was adopted by George Stephenson on several early railways before becoming widely established.

In modern usage, gauges narrower than Standard Gauge are generally described as narrow gauge, while wider gauges are described as broad gauge.

Larger scales offer impressive models with plenty of scope for detail. The PECO guide includes larger combinations such as SM-32, Gauge 1 and G-45, each offering a distinctive approach to large-scale railway modelling.

Narrow-gauge railways use a track gauge narrower than Standard Gauge. Because scale and gauge can be combined in different ways, narrow-gauge modelling covers a wide range of combinations, allowing modellers to represent metre-gauge, industrial, mining and other specialist railways.

Examples in the table include N-6.5, HOm, HOn3, OO-9, O-16.5, G-45 and SM-32, each combining a particular modelling scale with a narrower prototype gauge.