A surface warship became a cruiser if it exceeded 1,850 tons standard displacement or carried a gun above 5.1 inches. The cruiser category then split at 6.1 inches, separating the heavy and light subcategories. A destroyer stayed at or below both the 1,850-ton and 5.1-inch limits. These rules made displacement and gun calibre jointly decisive. The agreement permitted a transfer of up to 10 percent between light-cruiser and destroyer tonnage, although no navy used the option in practice.
Chapter · 12:06-13:46
Defining cruisers and destroyers by displacement and guns
The treaty drew a hard cruiser-destroyer boundary and permitted a transfer of up to 10 percent between light-cruiser and destroyer allowances.