Arabic distinguishes four pairs of consonants that differ only in a secondary articulation called emphasis, or in the terminology of the recitation manuals, heaviness. Sin and sad, dal and dad, ta and its emphatic counterpart, dhal and za: in each pair the second member is made with the same primary closure as the first while the back of the tongue is drawn up and back toward the soft palate.
Two things follow, and both matter for reading the script. The first is that emphasis is not confined to the consonant. It spreads to the vowels beside it and often across the whole syllable or word, so that the a beside an emphatic sounds markedly darker than the a beside a plain consonant. A reader hears the emphatic before reaching it. The second is that Arabic writes these as four separate letters with no shared skeleton between the members of a pair, which is a different strategy from the one the script uses for tha and ta or for jim and ha, where a single shape is split by dots. The emphatics were inherited as distinct letters from the parent alphabet, so no repair was needed.
In tajwid, the discipline that governs Quranic recitation, the heavy letters are grouped and named, and the rules for when neighbouring sounds are darkened by them are set out in detail. That body of rules is transmitted orally, under licence from a teacher whose own chain of transmission is named, and it records distinctions the written page does not: which letters are heavy, when a normally light letter darkens, and how long a sound is held. The script fixes the consonants and the tradition fixes the voice.