Saudi S. answered 02/25/22
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These are six letters أ د ذ ر ز و ز . They are known as the ‘kicking letters’ as they kick (do not join to) the following letters although they are joined to the previous letters unless these previous letters belong to this group. I myself call them ‘naughty letters’. Examples are provided in the previous answers, but here are examples when any naughty letter is preceded by another naughty letter:
وِزْر sin
زُور falsehood
زِرّ button
أَزْرَار buttons
أَرُزّ rice
رَذَاذ dribble
وُدّ affection
وَدُود affectionate