HO is the most popular narrow gauge modeling size or scale followed by O and S.
When an actual railroad item is
reduced to HO, the size of the model is 1/87 th of the
actual item. The following table lists other popular narrow gauge
combinations.  The dimension that identifies
narrow gauge modeling is the distance between the railroad track rails.

HOn30 is also known as
HOn2 1/2˝ or Hoe.  It is simply one way of
modeling narrow gauge track and trains. The parts of this term come from 'HO'
referring to HO scale, 'n' referring to narrow gauge and '30' referring to the
30 inch spacing between the railroad track rails (the gauge) of the prototype.
How this is modeled is explained below.

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. While it is true that an HO scale standard gauge locomotive runs on HO
gauge track, it does not follow that you can have a HO “gauge” village hall.
Both standard and narrow gauge HO locomotives and buildings are the same 3.5mm
on the model equals one real foot scale.

Assuming you are interested
in the way narrow gauge railroads look and operate, the easiest approach with
narrow gauge modeling is to select a commercially supported scale that results
in engines and track of your 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 models and N gauge locomotives and track. 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 math using the above figures it can
be seen that when N gauge track is used in HO scale it works out close to
30". Modelers use HOn30 to model gauges between 24" and 30".

The following table lists
other popular narrow gauge combinations.