Establishing it is perhaps the most essential aspect of the horseshoer's art. Many horse owners don't fully understand what hoof balance is. But they know that if their horses don't have it, they may suffer from poor movement, interference or lameness. Many good farriers don't seem to consciously know the technical features of hoof balance, but instead have developed a subconscious understanding of it through years of experience. This works fine for them when shoeing a horse, but doesn't allow them to explain balance to the owners.
Hoof balance is four-dimensional geometry.
The fourth dimension is time, which manifests as movement, growth and change. It's this fourth dimension that makes things tricky. Were it not for this dimension, anyone with good measuring tools and the ability to read them could ascertain balance. Somewhere along the line, long before the hoof protractor was invented, some fellow who probably had never shod horses decided that 45 degrees was the proper toe angle for front hooves. This erroneous idea has been passed down in books from around 1800 to the present.