Why carriers were a difficult treaty category

Aircraft carriers posed a particularly awkward problem for the Washington Treaty. Early carriers were few, experimental, and often incomplete, so negotiators had little reliable evidence for deciding what counted as a carrier or how much capability a tonnage allowance bought. The Royal Navy could classify nearly every carrier before Courageous as experimental, making replacement rules and treaty accounting unusually flexible. Conversion allowances created another difficulty because an inherited hull could consume more treaty tonnage for less aviation capacity than a purpose-built ship.