Hamza (Arabic: همزة, hamzah) (ء) is a letter in the Arabic alphabet, representing the glottal stop [ʔ]. Hamza is not one of the 28 "full" letters and owes its existence to historical inconsistencies in the standard writing system. It is derived from the Arabic letter ʿAyn. In the Phoenician and Aramaic alphabets, from which the Arabic alphabet is descended, the glottal stop was expressed by alif (), continued by Alif ( ) in the Arabic alphabet. However, Alif was used to express both a glottal stop and also a long vowel /aː/. In order to indicate that a glottal stop is used, and not a mere vowel, it was added to Alif diacritically. In modern orthography, hamza may also appear on the line, under certain circumstances as though it were a full letter, independent of an Alif. In Unicode it is at the code point U+0621 and named ARABIC LETTER HAMZA.
The letter Hamza on its own always represents hamzat qaṭ‘ (هَمْزَة الْقَطْع); that is, a phonemic glottal stop unlike the hamzat waṣl (هَمْزَة وَصْل), a non-phonemic glottal stop produced automatically at the beginning of an utterance. Although it can be written as Alif carrying a Waṣlah sign ٱ (only in the Quran), it is normally indicated by a plain Alif without a Hamza.
The rules for writing hamza differ somewhat between languages even if the writing is based on the Arabic abjad. The following addresses Arabic specifically.
Not surprisingly, the complexity of the rules causes some disagreement.
^[a] Arabic writing has tried to avoid two consecutive wāws, however, in Modern Arabic this rule is less applicable, thus modern رؤوس ruʾūs "heads" corresponds to رءوس in the Quran.
In Urdu script, hamza does not occur at the initial position over alif since alif is not used as a glottal stop in Urdu. In the middle position, if hamza is surrounded by vowels, it indicates a diphthong between the two vowels. In the middle position, if hamza is surrounded by only one vowel, it takes the sound of that vowel. In the final position hamza is silent or produces a glottal sound, as in Arabic.
In Urdu, hamza usually represents a diphthong between two vowels. It rarely acts like the Arabic hamza except in a few loanwords from Arabic.
Hamza is also added at the last letter of the first word of ezāfe compound to represent -e- if the first word ends with yeh or with he or over bari yeh if it is added at the end of the first word of the ezāfe compound.
Hamza is always written on the line in the middle position unless in waw if that letter is preceded by a non-joiner letter; then, it is seated above waw. Hamza is also seated when written above bari yeh. In the final form, Hamza is written in its full form. In ezāfe, hamza is seated above he, yeh or bari yeh of the first word to represent the -e- of ezāfe compound.