Ẓāʾ

Ẓāʾ, or ḏ̣āʾ (ظ), is one of the six letters the Arabic alphabet added to the twenty-two inherited from the Phoenician alphabet (the others being ṯāʾ, ḫāʾ, ḏāl, ḍād, ġayn). In name and shape, it is a variant of ṭāʾ. Its numerical value is 900 (see Abjad numerals).

Ẓāʾ ظَاءْ does not change its shape depending on its position in the word:

Pronunciation
In Classical Arabic, it represents a velarized voiced dental fricative, and in Modern Standard Arabic, it can also be a pharyngealized voiced dental or alveolar  fricative.

In most Arabic vernaculars ظ ẓāʾ and ض ḍād merged quite early. The outcome depends on the dialect. In those varieties (such as Egyptian, Levantine and Hejazi), where the dental fricatives and  are merged with the dental stops  and, ẓādʾ is pronounced  or  depending on the word; e.g. ظِل is pronounced  but ظاهِر is pronounced , In loanwords from Classical Arabic ẓāʾ is often , e.g. Egyptian ʿaẓīm (< Classical عظيم ʿaḏ̣īm) "great".

In the varieties (such as Bedouin and Iraqi), where the dental fricatives are preserved, both ḍād and ẓāʾ are pronounced. However, there are dialects in South Arabia and in Mauritania where both the letters are kept different but not consistently.

A "de-emphaticized" pronunciation of both letters in the form of the plain entered into other non-Arabic languages such as Persian, Urdu, Turkish. However, there do exist Arabic borrowings into Ibero-Romance languages as well as Hausa and Malay, where ḍād and ẓāʾ are differentiated.

Statistics
Ẓāʾ is the rarest phoneme of the Arabic language. Out of 2,967 triliteral roots listed by Hans Wehr in his 1952 dictionary, only 42 (1.4%) contain ظ. It is the only Arabic letter not used in any country names in Arabic.

In other Semitic languages
In some reconstructions of Proto-Semitic phonology, there is an emphatic interdental fricative, ṯ̣/ḏ̣ ( or ), featuring as the direct ancestor of Arabic ẓādʾ, while it merged with ṣ in most other Semitic languages, although the South Arabian alphabet retained a symbol for ẓ.

In relation with Hebrew
Often, words that have ظ ẓāʾ, ص ṣād, and ض ḍād in Arabic have cognates with צ tsadi in Hebrew.
 * Examples:
 * ظ ẓāʾ: the word for "thirst" in Classical Arabic is ظمأ ẓamaʾ and צמא tsama in Hebrew.
 * ص ṣād: the word for "Egypt" in Classical Arabic is مصر miṣr and מצרים mitsrayim in Hebrew.
 * ض ḍād: the word for "egg" in Classical Arabic is بيضة bayḍah and ביצה betsah in Hebrew.

When representing this sound in transliteration of Arabic into Hebrew, it is written as ט׳ tet and a geresh or with a normal ז zayin.