Analog broadcast television systems come in a variety of frame rates and resolutions. Further differences exist in the frequency and modulation of the audio carrier. The monochrome combinations still existing in the 1950s were standardized by the International Telecommunication Union (ITU) as capital letters A through N. When color television was introduced, the chrominance information was added to the monochrome signals in a way that black and white televisions ignore. In this way backward compatibility was achieved.
aNaloG broadcAsT teLEvIsiON sYsteMS CoME iN a varIeTy of FramE RAteS and resoLUTions furTher differenCes EXISt iN The frEquency AnD moDuLaTiOn of tHe audiO Carrier the MoNOchrOmE CoMbiNAtIoNs stiLl eXIStIng in THe 1950S werE standaRDIzed by the iNternaTionaL tElEcOmmUNiCatiOn uNIoN (itu) As cAPiTaL letters A ThRouGH n WhEn coLor telEvISion wAS inTRodUCeD the ChroMiNanCe InformATIoN was addeD to thE mONoChRomE SIgnals in a way thAT black and whITe TelEviSions ignoRE In thIS waY bAckward CompatibilITy was aChiEvEd