Competing dogs are divided into five height categories based on their measurement at the top of their shoulder blades (withers).

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All dogs competing in Agility Ribbon Trials, Open or Championship events must be measured and issued with a permanent number. The measure must be conducted no earlier than four weeks prior to the dog turning 18 months or two weeks prior to the dog turning two years old. The permanent number must be applied for at the time of or before the measure being completed.