What is an abjad — Abjad and Vowel Marks

An abjad is a writing system in which the letters stand for consonants, and the vowels are supplied by the reader — or by small optional marks.

The label is an acronym of the old Arabic letter order — alif, ba, jim, dal — just as 'alphabet' spells out alpha and beta. As a technical term it is recent: the linguist Peter T. Daniels proposed it in 1990, so that consonantal scripts would no longer have to borrow the name of the vowel-writing alphabets.

The defining rule is about what is obligatory. Every consonant of the word must be written, because the consonants are the word. Vowels, by contrast, are facultative: they can be left out entirely, hinted at with a consonant letter used as a vowel carrier, or spelled out with small marks — and none of that changes the underlying letters.

That is what separates an abjad from an alphabet. When Greek scribes adapted the Phoenician letters around the ninth or eighth century BCE, they reassigned a few consonant signs to vowel sounds, and from then on vowels were full letters written in the line. An abjad never takes that step: its vowels stay outside the letter inventory.

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It also separates an abjad from a syllabary, where each sign stands for a whole syllable, and from an abugida, where each consonant sign carries an inherent vowel that attached marks modify. In an abjad the sign is a bare consonant and the vowel is the optional layer.

The design fits the languages that use it. Semitic words are built on consonantal roots — k-t-b carries the idea of writing in Arabic — and a small set of vowel patterns turns the root into actual words. A fluent reader knows the patterns, so the unwritten vowels are predictable most of the time.

Abjads are not a historical curiosity. Arabic is the everyday script of hundreds of millions of readers, Hebrew is read unpointed across Israel, and Syriac remains in liturgical use — all of them consonantal systems in which the vowels live mostly in the reader's head.

Consonant letters are obligatory — drop one and the word is gone.
Vowels are facultative — unwritten, hinted, or fully marked.
Matres lectionis are consonants moonlighting as long-vowel carriers.
The historical letter order doubles as a number system.
The major abjads are written right to left.