The roots of verbs and most nouns in the Semitic languages are characterized as a sequence of consonants or "radicals" (hence the term consonantal root). Such abstract consonantal roots are used in the formation of actual words by adding the vowels and non-root consonants (or "transfixes"), which go with a particular morphological category around the root consonants, in an appropriate way, generally following specific patterns.

It is a peculiarity of Semitic linguistics that many of these consonantal roots are triliterals, meaning that they consist of three letters (although there are a number of quadriliterals, and in some languages also biliterals). Such roots are also common in other Afroasiatic languages. While Berber mostly has triconsonantal roots, Chadic, Omotic, and Cushitic have mostly biconsonantal roots; and Egyptian shows a mix of biconsonantal and triconsonantal roots.

A triliteral or triconsonantal root (Arabic: جذر ثلاثي, jiḏr ṯulāṯī; Hebrew: שורש תלת־עיצורי, šoreš təlat-ʻiṣuri; Syriac: ܫܪܫܐ, šeršā) is a root containing a sequence of three consonants.

The following are some of the forms which can be derived from the triconsonantal root k-t-b כ־ת־ב ك-ت-ب (general overall meaning "to write") in Hebrew and Arabic:

The Hebrew fricatives stemming from begadkefat lenition are transcribed here as "ḵ", "ṯ" and "ḇ", to retain their connection with the consonantal root כ־ת־ב k-t-b. They are pronounced [x], [θ], [β] in Biblical Hebrew and [χ], [t], [v] in Modern Hebrew respectively. Modern Hebrew has no gemination; where there was historically gemination, they are reduced to single consonants, with consonants in the begadkefat remaining the same.

In Hebrew grammatical terminology, the word binyan (Hebrew: בניין, plural בניינים binyanim) is used to refer to a verb derived stem or overall verb derivation pattern, while the word mishqal (or mishkal) is used to refer to a noun derivation pattern, and these words have gained some use in English-language linguistic terminology. The Arabic terms, called وزن wazn (plural أوزان, awzān) for the pattern and جذر jaḏr / jiḏr (plural جذور, juḏūr) for the root have not gained the same currency in cross-linguistic Semitic scholarship as the Hebrew equivalents, and Western grammarians continue to use "stem"/"form"/"pattern" for the former and "root" for the latter—though "form" and "pattern" are accurate translations of the Arabic grammatical term wazan (originally meaning 'weight, measure'), and "root" is a literal translation of jiḏr.

Although most roots in Hebrew seem to be triliteral, many of them were originally biliteral, cf. the relation between:

The Hebrew root ש־ק־ף‎ – √sh-q-p "look out/through" or "reflect" deriving from ק־ף‎ – √q-p "bend, arch, lean towards" and similar verbs fit into the shaCCéC verb-pattern.^{[clarification needed]}