A cedilla is a hook (¸) added under certain consonant letters as a diacritic mark to modify their pronunciation. The tail is the bottom half of a miniature cursive z or Ezh: Ʒ/ʒ (). The name "cedilla" is the diminutive of the old Spanish name for the letter Z, ceda. An obsolete spelling of "cedilla" is "cerilla" because the letters d and r were interchangeable in 16th-century Spanish.

The most frequent character with cedilla is the ç (c with cedilla). This letter was used for the sound of the affricate [ts] in old Spanish. Spanish has not used it since an orthographic reform in the 18th century.

C-cedilla was adopted for writing other languages, like Portuguese, Catalan, unofficial Basque, Occitan, and some Friulian dialects, where it represents /s/ where "c" would normally represent /k/ (for example, while ca is normally pronouced as /ka/, ça is pronouced as /sa/); or Turkish, Azerbaijani, Tatar, Turkmen, Kurdish (at least the Mahabad dialect), and some Friulian dialects, where it is used for the sound of the affricate [tS] (the same of English in church). It is also used in a Romanization of Arabic. As a phonetic symbol, [ç] is the International Phonetic Alphabet symbol for the voiceless palatal fricative.

And the s-cedilla, ş, represents /S/ (as in show) in Turkish, Azerbaijan, Tatar, Turkmen, and Kurdish. It is also used in some Romanizations of Arabic, Persian, and Pashto, for the letter ar_r.

In the International Phonetic Alphabet, ç represents the voiceless palatal fricative.

In the Turkish alphabet both Ç and Ş are considered separate letters, not variants of C and S.

A few words are sometimes spelled in English with a ç, almost all of them borrowings from French, for example façade, soupçon and garçon.

The Romanian Ș (ș) seemingly resembles the Turkish s cedilla, but it is actually a comma (Virgula). While it is common in online contexts to use Ş/ş and Ţ/ţ in writing Romanian, that is only because they look almost right and are much more widely supported in character sets. The orthographically correct characters are Ș/ș and Ț/ț (may not appear on your browser).

Unicode considers Romanian S with comma (Virgula) below to be a glyph variant of S with cedilla (compare Han unification, where similar-looking Chinese characters from various languages or countries were merged), and intends the same character to be used for Turkish s-cedilla and Romanian s-comma, and "t-cedilla" to be used for Romanian t-comma. Since no European language has a letter t with cedilla, the letter looks like a t with a comma in many fonts, suitable for writing Romanian. Only later (in version 3.0) did Unicode add separate code points for s with comma and t with comma; these points have the comment "Romania, when distinct comma below form is required", while the code points with cedilla have the comments "this character is used in both Turkish and Romanian data" and "a glyph variant with comma below is preferred for Romanian" (similar to how LATIN SMALL LETTER D WITH CARON ď is marked "the form using apostrophe is preferred in typesetting").

It is, therefore, debatable whether Ş/ş and Ţ/ţ are "wrong"; if text is marked as being in the Romanian language, glyphs appropriate for Romanian should be chosen. This is similar to how language determines the glyph shape for Chinese characters (see Latvian letters g, k, l, n, and formerly r are considered by some to be cedillas and by others to be commas.