At first, Ingo indicated an emperor who left his throne while he was alive, but it became a custom to confer Ingo as a posthumous title on the Emperor who passed away during his reign.
当初は存命中に譲位した天皇を指すものであったが、在位中に崩御した天皇に対しても追号として院号を贈る慣例が出来るようになった。