## Original Question:

Model cars (of the Dinky Toy type) are on a scale of 1/43. 43 is a prime number. Who chose that scale? Why not 1/50 or 1/40? Is it related to the Imperial system of measures? How?

## Some partial answers:

Frode Tennebø sent me several links with extra info: it seems that the scale for model cars was taken from that of model railways so they would fit together. It's interesting and no doubt true, but it just moves the question one level further back in history: why 1/43 for model railways? To answer that, Frode sent these interesting links: O-scale and Model railway.

Yves Vanwetswinkel sent a link to a model railway site (in Dutch) where you can find a scale of 7mm to 1 foot. That's almost 1/43, but again it just displaces the question: who came up with using 7mm for one foot?

Dominik Buss found what is to me the most convincing history on a German model railway site where the story goes roughly as follows:

The original track width of 4'8.5" came from an old mining railway of 4'8", widened by 0.5" to manage going around bends (an interesting trick, explained elsewhere). The real track is then 56.5" wide. Making a model railway and choosing a model track width of 5" gave a scale of 5/56.5 = 1/11.3. With better machinery the models could be made smaller and smaller, choosing a model track of 2", then 1.75" and finally 1.25" would give a scale of 1/45.2

Note that apparently it was considered important that the model track width was a simple number, not the scale! The strange part of the explanation is that 1/45.2 was too difficult and choosing 7mm for a foot was considered the best way of calculating the reductions.

But I still find that amazing! Quite clearly reducing measures requires calculation and that happens in decimal. By the time high-precision tools became widely available, the inch had been divided into decimal parts (1/1000th or "mil") to allow for easier calculations. So I still don't understand why 7mm or, more important, why the track width was considered more important than the scale.

The first models were at scale 1/6 or 1/8. Those would be easy scales in the Imperial system of measures. 1/12 also, 1 inch to the foot. But 7mm? Or 1 inch to the track?

Once a manufacturer has success with a product, the scale becomes fixed because of compatibility, and from that point on all other manufacturers are almost obliged to follow. That explains why we are stuck with 1/43 and HO, but not how it came into being.

Maybe it was just some bad choice someone made?