Firstly, when discussing narrow gauge models it is important to make the distinction between scale and gauge. These terms can tend to be used interchangeably by those who model only standard gauge. Whilst it is true that a HO scale standard gauge locomotive runs on HO gauge track, it does not follow that you can have a HO gauge village hall!

Assuming you do not wish to do everything the hard way, the trick with narrow gauge modelling is to select a commercially supported scale that results in track of the correct gauge being available and commercially supported. Although that sounds a bit hit and miss it does actually work out quite well.

HOn30 works by combining HO scale and N gauge;
HO scale is 3.5mm per foot or 1:87. [HO Gauge is 16.5mm].
N gauge is 9mm [N scale is 1:148(UK) or 1:160(US), close to 2mm per foot].
Working through the maths using the above figures it can be seen that when N gauge track is used in HO scale it works out close to 30". In practice HOn30 is used to model gauges between 24" and 30".

The following table lists other combinations that are close to HOn30 in either scale or gauge;

Name	mm/foot	Gauge(mm)	Description
009	4	9	Scales out to 2' 3" gauge, popular in UK
00n12	4	12	Used to model 3' gauge prototypes
HOn3	3.5	10.5	Used to model 3' gauge, mainly in US
Sn2	4.8	9 or 10.5	Used for 2' gauge prototypes
HOm	3.5	12	Used in Europe to model metre gauge.