Hi Beman, Hi Howard, With current implementation of the metafunction ratio_subtract I get the type from the test ratio_test.cpp User1::Distance d( User1::mile(110) ); User1::Time t( boost::chrono::hours(2) ); typeof (d/t) is User1::quantity<boost::ratio<1,-1>, boost::ratio<1,1> > So the following assignation do not works User1::Speed s = d / t; as User1::Speed is defined as typedef quantity<boost::ratio<1>, boost::ratio<0> > Time; // second typedef quantity<boost::ratio<0>, boost::ratio<1> > Distance; // meter typedef quantity<boost::ratio<-1>, boost::ratio<1> > Speed; // meter/second The code is a little bit triky, and I have not taken the time to understand why the resulting type is User1::quantity<boost::ratio<1,-1>, boost::ratio<1,1> > and not <boost::ratio<-1,1>, boost::ratio<1,1> > User1::quantity I have replaced the type by typedef ratio<R1::num * R2::den - R2::num * R1::den,R1::den * R2::den> aux_type; typedef ratio<aux_type::num ,aux_type::den > type; // get normalized type to follow the recommendation "The nested typedef type shall be a synonym for ratio<T1, T2> where T1 has the value R1::num *R2::den - R2::num * R1::den and T2 has the value R1::den * R2::den." and every thing is OK (See below the replaced code) I'm doing something wrong? Best, _____________________ Vicente Juan Botet Escribá template <class R1, class R2> struct ratio_subtract { private: #if 1 typedef ratio<R1::num * R2::den - R2::num * R1::den,R1::den * R2::den> aux_type; #else static const boost::intmax_t gcd_n1_n2 = detail::static_gcd<R1::num, R2::num>::value; static const boost::intmax_t gcd_d1_d2 = detail::static_gcd<R1::den, R2::den>::value;#endif public: //The nested typedef type shall be a synonym for ratio<T1, T2> where T1 has the value // R1::num *R2::den - R2::num * R1::den and T2 has the value R1::den * R2::den. #if 1 typedef ratio<aux_type::num ,aux_type::den > type; #else typedef typename ratio_multiply < ratio<gcd_n1_n2, R1::den / gcd_d1_d2>, ratio < detail::ll_sub < detail::ll_mul<R1::num / gcd_n1_n2, R2::den / gcd_d1_d2>::value, detail::ll_mul<R2::num / gcd_n1_n2, R1::den / gcd_d1_d2>::value >::value, R2::den > >::type type; #endif };