Because of the Russian system of calling types and sub-types. Eg... Yak-1 was the base, then Yak 1B for instance. These letters after the digit means that there was someting improved or modified or means the purpose it was designed for, eg... Yak-9DD or Yak-9T. The "DD" means "Dalnego Diejstwija" - long range and the "T" - tjażolyj - heavy. If there was no change introduced there wasn't any letter after the name of the type.
Because of the Russian system of calling types and sub-types. Eg... Yak-1 was the base, then Yak 1B for instance. These letters after the digit means that there was someting improved or modified or means the purpose it was designed for, eg... Yak-9DD or Yak-9T. The "DD" means "Dalnego Diejstwija" - long range and the "T" - tjażolyj - heavy. If there was no change introduced there wasn't any letter after the name of the type.
