Summary

• Initial hamza is always placed over (أ for ʾa- or ʾu-) or under (إ for ʾi-) an alif.
• Medial hamza will have a seat or be written alone:
  • Surrounding vowels determine the seat of the hamza with preceding long vowels and diphthongs (such as aw or ay) being ignored.
  • i- (ئ) over u- (ؤ) over a- (أ) if there are two conflicting vowels that count; on the line (ء) if there are none.
  • As a special case, āʾa, ūʾa and awʾa require hamza on the line, instead of over an alif as one would expect. (See III.1b below.)
• Final hamza will have a seat or be written alone:
  • Alone on the line when preceded by a long vowel or final consonant.
  • Has a seat matching the final short vowel for words ending in a short vowel.
• Two adjacent alifs are never allowed. If the rules call for this, replace the combination by a single alif maddah.

Detailed description

• Logically, hamza is just like any other letter, but it may be written in different ways. It has no effect on the way other letters are written. In particular, surrounding long vowels are written just as they always are, regardless of the "seat" of the hamza—even if this results in the appearance of two consecutive wāws or yāʾs.
• Hamza can be written in five ways: on its own ("on the line"), under an alif, or over an alif, wāw, or yāʾ, called the "seat" of the hamza. When written over yāʾ, the dots that would normally be written underneath are omitted.
• When according to the rules below, a hamza with an alif seat would occur before an alif which represents the vowel ā, a single alif is instead written with the maddah symbol over it.
• The rules for hamza depend on whether it occurs as the initial, middle, or final letter (not sound) in a word. (Thus, final short inflectional vowels do not count, but -an is written as alif + nunation, counts, and the hamza is considered medial.)

I. If the hamza is initial:

• If the following letter is a short vowel, fatḥah (a) (as in أَفْرَاد‎ ʾafrād) or ḍammah (u) (as in أُصُول‎ ʾuṣūl), the hamza is written over a place-holding alif; kasrah (i) (as in إِسْلَام‎ ʾislām) the hamza is written under a place-holding alif and is called "hamza on a wall."
• If the letter following the hamza is an alif itself: (as in آكُل‎ ʾākul) alif maddah will occur.

II. If the hamza is final:

• If a short vowel precedes, the hamza is written over the letter (alif, wāw, or yāʾ) corresponding to the short vowel.
• Otherwise, the hamza is written on the line (as in شَيْء‎ shayʾ  "thing").

III. If the hamza is medial:

• If a long vowel or diphthong precedes, the seat of the hamza is determined mostly by what follows:

• If i or u follows, the hamza is written over yāʾ or wāw, accordingly.
• Otherwise, the hamza would be written on the line. If a yāʾ precedes, however, that would conflict with the stroke joining the yāʾ to the following letter, so the hamza is written over yāʾ. (as in بطِيئَة‎)

• Otherwise, both preceding and following vowels have an effect on the hamza.

• If there is only one vowel (or two of the same kind), that vowel determines the seat (alif, wāw, or yāʾ).
• If there are two conflicting vowels, i takes precedence over u, u over a so miʾah 'hundred' is written مِئَة‎, with hamza over the yāʾ.
• Alif-maddah occurs if appropriate.

Not surprisingly, the complexity of the rules causes some disagreement.

• Barron's 201 Arabic Verbs follows the rules exactly (but the sequence ūʾū does not occur; see below).
• John Mace's Teach Yourself Arabic Verbs and Essential Grammar presents alternative forms in almost all cases when hamza is followed by a long ū. The motivation appears to be to avoid two wāws in a row. Generally, the choice is between the form following the rules here or an alternative form using hamza over yāʾ in all cases. Example forms are masʾūl (مَسْئُول‎, [adj: responsible, in charge, accountable]; [noun: official, functionary]), yajīʾūna (يَجِيئُونَ‎, verb: jāʾa جَاءَ‎ "to come"), yashāʾūna (يَشَائُونَ‎, verb: shāʾa شَاءَ‎ "to will, to want, to intend, to wish"). Exceptions:

• In the sequence ūʾū (yasūʾūna, verb: sā'a سَاءَ‎ "to act badly, be bad") the alternatives are hamza on the line يَسُوءُونَ‎, or hamza over yāʾ يَسُوئُونَ‎, when the rules here would call for hamza over wāw. Perhaps, the resulting sequence of three wāws would be especially repugnant.
• In the sequence yaqraʾūna (يَقْرَأُونَ‎, verb: qaraʾa قَرَأَ‎ "to read, to recite, to review/ study") the alternative form has hamza over alif, not yāʾ.
• The forms yabṭuʾūna (يَبْطُؤُونَ‎, verb: baṭuʾa بَطُؤَ‎ "to be or become slow, late or backward, "to come late", "to move slowly") and yaʾūbu (يَؤُوبُ‎, verb: آبَ‎ "move to the back", "to return to come back", "to repent") have no alternative form. (Note yaqraʾūna with the same sequence of vowels.)

• Haywood and Nahmad's A new Arabic Grammar of the Written Language does not write the paradigms out in full, but in general agrees with John Mace's book, including the alternative forms and sometimes lists a third alternative with the entire sequence ʾū written as a single hamza over wāw instead of as two letters.
• Al-Kitāb fī Taʿallum... presents paradigms with hamza written the same way throughout, regardless of the rules above. Thus yabdaʾūna with hamza only over alif, yajīʾūna with hamza only over yāʾ, yaqraʾīna with hamza only over alif, but that is not allowed in any of the previous three books. (It appears to be an overgeneralization on the part of the al-Kitāb writers.)

Jawi alphabet

In the Jawi alphabet (Arabic script used to write Malay), hamza is used for various purposes, but is rarely used to denote a glottal stop except in certain Arabic loanwords. The default isolated hamza form (Malay: hamzah setara) is the second least common form of hamza,^[4] whereas another form unique to the Jawi script, the three-quarter high hamza (Malay: hamzah tiga suku) is most commonly used in daily Jawi writing. The three-quarter high hamza itself is used in many cases:^[6]

• Separating vowel letters of a diphthongsuch as ai, au, and oi when present in certain positions within words
• Preceding certain suffixes such as ⟨ن‎⟩ (-an) and ⟨ي‎⟩ (-i)
• To write non-Malay single-syllable words (most commonly names) that starts with a vowel other than alif ⟨ا⟩
• Glottal stops for archaic words (specifically titles and names which have a fixed spelling)
• In some instances Arabic loanwords which change their original spelling may change the hamza to the three-quarter high hamza instead

This exact form is not available in Unicode Standard, as it is unified with ARABIC LETTER HIGH HAMZA, but the common way of writing this form is by using a normal hamza and altering its vertical position.

Hamza above alif ⟨أ‎⟩ is used for prefixed words using the prefixes ⟨ک‎⟩, ⟨د‎⟩, or ⟨س‎⟩, where its root word starts with a vowel (such as د+امبيل‎ (di+ambil), becomes دأمبيل‎ (diambil)). This form as well as hamza below alif ⟨إ‎⟩ are both also in Arabic loanwords where the original spelling has been retained.

The hamza above ya ⟨ئ‎⟩ is known as a "housed hamzah" (Malay: hamzah berumah), and is most commonly used in Arabic loanwords. It is also used for words which repeat or combine "i" and "é" vowels like چميئيه‎ (cemeeh meaning "taunt") and for denoting a glottal stop in the middle of a word after a consonant such as سوبئيديتور‎ (subeditor). More commonly, however, it is used for denoting a schwa after the vowels "i", "é", "o", and "u" such as چندليئر‎ (chandelier).

Hamza above waw ⟨ؤ‎⟩ is completely removed from the Jawi alphabet, and for Arabic loanwords using the letter, it is replaced with a normal waw followed by a three-quarter high hamza instead.^[7]

Urdu (Shahmukhi) alphabet

In the Urdu alphabet, 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.

Uyghur alphabet

In the Uyghur Arabic alphabet, the hamza is not a distinct letter and is not generally used to denote the glottal stop, but rather to indicate vowels. The hamza is only depicted with vowels in their initial or isolated forms, and only then when the vowel starts a word. It is also occasionally used when a word has two vowels in a row.^[8]

Kazakh alphabet

In the Kazakh Arabic alphabet, the hamza is used only at the beginning of words, and the only form is high hamza. It is not used to denote any sound, but to indicate that the vowels in the word will be the four front vowels: ⟨ٵ‎⟩ (ä), ⟨ٸ‎⟩ (ı), ⟨ٶ‎⟩ (ö), ⟨ٷ‎⟩ (ü). However, it is not used for words containing another front vowel ⟨ە‎⟩ (e) or words containing four consonants ⟨گ‎⟩ (g), ⟨غ‎⟩ (ğ), ⟨ك‎⟩ (k), ⟨ق‎⟩ (q).^[9]