This is not the binary opposite of its partner in the “Glottal Closure” drawer because singing is not a binary system. IGC is a controlled departure from Complete Glottal Closure (CGC). IGC covers a universe of differences from CGC which reach the epitome of different when one manages to breathe silently.
The softest whisper to the loudest of “Head Voice” singing can be categorized as IGC. There are pathologies that can make IGC the only function possible to the Vocal Folds, but these are not part of the discussion.
Mastering the use of IGC is the final level, the very top of making artistic interpretations.
