At a cat show, the judges compare each cat to the standard for its breed. The Cat Fancier’s Association (CFA) publishes an established standard for each breed. These standards describe an ideal example of the breed, and assign point values to different features to help establish their relative importance. For instance, the standard may indicate that the head is overall worth 30 points (with shape worth 15, the ears 10, and the eyes five).
Judges also look for overall balance and proportion in a cat’s features, rather than exactly conforming mathematical measurements. Temperament also plays an important part in judging cats. If a cat eschews the spotlight, winning will be much harder, despite its physical attributes.
The standards and point values differ for each breed, and judges must be familiar with a wide breadth of information to judge different breeds against each other. Many cat shows also incorporate a household pet category, discussed here.
Occasionally, the CFA’s breed council will propose changes to breed standards as breeds develop. You can find standards for all CFA-recognized breeds here.