Why are the letters of Izhar the letters of Izhar? On what basis are they categorized as the letters of Izhar/Idhar?

Question

Why are the letters of Izhar the lettesr of Izhar ? On what basis are they categorized as the letters of Izhar/Idhar?

Answer
Assalaam alaikum,
The letters of    are called thus because they cause the previous letter to be pronounced clearly.  In the case of the saakinah, when the letters pronounced from throat , follow the saakinah, the is then pronounced clearly, which is what means.  Any other letter following the saakinah besides these six letters of cause a different rule to be used on the saakinah, and it is not pronounced clearly then. 

Please also see: www.abouttajweed.com/the-noon-saakinah-and-tanween/index.php
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