The name H0 is derived from the fact that its 1:87 scale is approximately half that of 0 scale which was the smallest of the series of older and larger 0, 1, 2 and 3 scales introduced by Märklin around 1900. In English H0 is pronounced "aitch-oh" (letters H O) but in German "hah-null" (letter H numeral 0). After the First World War there were several attempts to introduce a model railway about half the size of 0 scale that would be more suitable for smaller home layouts and cheaper to manufacture. H0 was created to meet these aims. For this new scale, a track width of 16.5 mm was designed to represent prototypical standard gauge track, and a model scale of 1:87 was chosen.
I don't know about the other scales. There are about 35 of them.
Bone has a good description of the two main scales 'O' and 'HO' . ( Although some items still defy logic - if HO was truly HALF of O then it SHOULD be 1/96, as above 'O' scale is 1/24 (called half scale) and 1/12 called inch scale. ) Here perhaps we begin to see the differences in all model scales partly because of the metric and foot based systems. There is still considerable controversy among model fanatics as to whether 'O' is intended to be 1/43 or 1/48 . In North America 'O' is considered 1/48 , in Europe it's 1/43 . Aircraft scales are similarly confusing ,about the commonest is 1/72, yet in France and Germany (major model producing countries) their equivalent is 1/75 ! - Model vehicle lines also had something to do with this, and possibly one of the more sensible scales is 1/76, the most popular small military vehicle scale. (chosen so that an average male human figure would be an inch tall ) -For practical purposes many scales can be used together, for instance, 1/72 figures with 1/76 vehicles and 54mm figures with 1/32 aircraft and military vehicles.
In conclusion, the whole scale question is bewildering to the layman, and as an ex-professional modelmaker my advice is to use 70 inches as an average human height and scale your models accordingly.
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