Until 1940 the armed forces adhered strictly to the convention that engines built for the Army Air Forces used odd numeric suffixes (e.g.: -5), while those built for the US Navy used even (e.g.: -8). After 1940, however, in the interests of standardization, engines were sometimes built to a joint Army-Navy contract, in which case the engines used a common numeric suffix (e.g.: the -10 was used by both Army and Naval aircraft.)