HO vs H0, OO vs 00, Etc...

 Recently, I began using H0 in my annotations. 

Look again. That's "aitch zero", not "aitch oh". 

Why, you ask?

Well, you see, the original designations for model railroad gauges were numerical, with 3 for 2.5", 2 for 2", 1 for 1.75", and 0 for 1.25". In the world of toy and model railroading, there was no incremental step to the next smaller commercial gauge. 

It was a leap - 00, which was idealized as half of 0, but worked out to be 0.65", or 16.5mm. This was followed in time by 000, 2mm to the foot, 9mm gauge. 

Then things started getting messy.

Just before the Second World War, in an attempt to make more scale like models but keep costs under control, American Flyer and Marx began making 3/16" to the foot models that ran on 0 gauge track. After the war, Marx continued this practice, but American Flyer switched to 7/8" gauge, and thus S gauge was born.

A letter designation has now entered the fray. 

Prior to that there were attempts to make more scale like rolling stock on 00 track. Doing the math, it was found that the gauge worked out closer to 1/87 scale than the larger scales (variously 1/80 to 1/70) used on 00. To differentiate these finer scale models the designation H0 was chosen.

With the postwar boom in smaller scales, especially H0, it was inevitable that someone, somewhere, would go even smaller. 

In 1945, Hal Joyce introduced TT gauge in the United States, 1/120 scale, 0.472" gauge. While it never really caught on in the US, it did become quite popular in Europe.

So now we have three model railroad sizes with letters - S, H0, and TT.

In time, they would be followed by even more letters - N, Z, T on the small end of the scale spectrum, and G on the larger. 

Naturally, there is very little visual difference betwixt O (oh) and 0 (zero), and in many places "oh" and "zero" are interchangeable in things such as counting (though of course not spelling). The transition of 0 to O, 00 to OO, and H0 to HO was really inevitable. 

Yet I now find some odd comfort in using H0 ("aitch zero"). It's legacy remains in that designation.

I suspect I will vacillate between HO and H0, being as I am only human, and a curious one at that.