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Originally devised by the Model Railway Study Group in the early 1960's, these standards were designed to follow prototype practice as closely, as practically and as economically as possible at the time.
Born from a desire to create better models, it was realised that by ensuring that the foundation of the wheels and rail were correctly proportioned, the need to make compromises would be greatly reduced, even disappear. The result of this is that models are proportioned much more as the prototype. This point is made very clear by the many cruelly enlarged photographs one regularly sees published in various journals.
Although adherence to the wheel and track standards is the fundament of the Proto approach, most Proto modellers these days will tell you that their goal is more about achieving realistic proportions in every aspect of the model, and not building to "exact scale".
Exact Scale implies just that, but we have to accept that we cannot scale the manufacturing tolerances as used in the prototype. Proto standards distinguish themselves by recognizing this and incorporating realistically achievable tolerances into the fully integrated standard. Adherence to one proven standard ensures success and ensures successful interchange between different layouts.