The Heavy letters are collected in the phrase:
The heavy letters have the quality of Isti’laa “rising high”. because while pronouncing them, a part of the tongue (mostly the back part) rises up to the roof makes the letter sound heavy.
All the letters other than the heavy letters are Light letters and temporary heavy letters — The Light Letters have the quality of Istifaal “falling down”, by lowering the tongue when pronouncing the light letter.
