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===== An Overview of the Main Narrow Gauge Scale/Gauge Combinations in the UK =====

There are many, many Narrow Gauge/Scale combinations. In fact there is probably a scale/gauge combination being used by someone for every scale in combination with almost every commercial track gauge and with many non-commercial ones. There are also scale/gauge combinations that use scales that don’t really exist in any form in standard gauge modelling. 

To understand what is meant by a particular narrow gauge title it is worth looking at the typical notation used. In most cases, and there are some exceptions to complicated the matter, in is described, as an example, as 09, so the scale followed by the gauge of the track. In many cases small letter n is inserted between the two, but not always. To complicate matters the gauge of the track sometimes refers to the model gauge, certainly the most common in the UK, and sometimes the gauge of the prototype track, more common in the USA and Europe.

Here we will look at the three main scale/gauge combinations in use in the UK, based around O Scale and OO Scale. The table below also includes the larger narrow gauge combinations, often used in garden railways.

**On16.5** is , with 009, a very widely used scale/gauge combination in the UK. Benefiting from the vast array of OO mechanism there is a very wide range of specialist support. It scales to a 2’ 4” prototype but is commonly used for 2’ to 2’ 6” and beyond with much success. Supported by the 7mm Narrow Gauge Association there is a lot of support available in terms of advice and parts.

**On9** is not as common as the other two scales covered here but due to the commercially available mechanisms from N gauge, and good specialist support through the trade and the 7mm Narrow Gauge Association.  The biggest restricting factor is probably the fact that it represents a 1’ 3” prototype and there were not so many of them and certainly not as well known as the wider gauge prototypes.

**009** is, with O-16.5, the most common narrow gauge combination in the UK. Strictly 4mm scale on 9mm track provides for a prototype of 2’ 3” gauge. However, it is often used to cover 2’ to 2’ 6” prototypes, and often larger still. Historically it has been a kit and scratch builders scale, albeit able to call on N scale mechanisms, but in the last few years number of the major commercial suppliers have released ready-to-run locomotives and rolling stock, increasing the take up considerably. The 009 Society is well established and provides a lot of support via the forum and the kits and parts it makes available to members. 

=== Summary of the main Scale/Gauge combinations ===
^ Scale   ^ Ratio  ^ Gauge   ^ Prototype Gauge ^ Notes ^
| G      |  1:22.5 | 45mm  | metre | A major scale, widely used as a garden railway |
| Gn15      |  1:22.5 | 45mm  |15”| Limited specialist support |
| SM32| 1:19 | 32mm | 2’ | Supported by commercial suppliers, primarily a garden scale |
| O-21 | 1:43.5 | 21mm | 3’ | Limited specialist support |
| **O-16.5**  | **1:43.5** | **16.5mm** | **2’ 3”** | Society and specialist trade support using standard track |
| O-14 | 1:43.5 | 14mm | 2’ | A niche combination for those looking for true 2’ gauge. Very limited trade support |
| **O9** | **1:43.5** | **9mm** | **15”** | Society and specialist trade support using standard track |
| On3 | 1:48 | 16.5mm | 3’ | Widely supported for USA prototypes |
| On30 | 1:48 | 14mm | 2’ 6” | Sometimes called On2½ mostly used for USA prototypes where it has major trade support |
| On2 | 1:48 | 12mm | 2’ | Limited specialist support, many use On30 to represent 2’ gauge prototypes |
| 5.5mm |   | 12 & 16.5mm | 2' & 3' | A supporting Society in the UK with limited trade support but using standard track gauges |
| Sn3 | 1:64 | 14mm  | 3’ | Limited specialist support |
| 00n3 | 1:76 | 12mm | 3' | Limited specialist trade support but is to the common OO scale|
| **009**    | **1:76** | **9mm** | **2’ 3”** |Commercial and specialist trade support with a large Society for support |
| HOm | 1:87 | 12mm | Metre| A major scale in Europe |
| HOe | 1:87 | 9mm | 60-75cm | Called HOn30 in the USA, a major scale in both the USA and Europe  |
| HOn3 | 1:87 | 10.5mm | 3’ | A major scale in both the USA|