Alexandrine is a name used for several distinct types of verse line with related metrical structures, most of which are ultimately derived from the classical French alexandrine. The line's name derives from its use in the Medieval French Roman d'Alexandre of 1170, although it had already been used several decades earlier in Le Pèlerinage de Charlemagne. The foundation of most alexandrines consists of two hemistichs (half-lines) of six syllables each, separated by a caesura (a metrical pause or word break, which may or may not be realized as a stronger syntactic break):

However, no tradition remains this simple. Each applies additional constraints (such as obligatory stress or nonstress on certain syllables) and options (such as a permitted or required additional syllable at the end of one or both hemistichs). Thus a line that is metrical in one tradition may be unmetrical in another.

Where the alexandrine has been adopted, it has frequently served as the heroic verse form of that language or culture, English being a notable exception.

The term "alexandrine" may be used with greater or lesser rigour. Peureux suggests that only French syllabic verse with a 6+6 structure is, strictly speaking, an alexandrine. Preminger et al. allow a broader scope: "Strictly speaking, the term 'alexandrine' is appropriate to French syllabic meters, and it may be applied to other metrical systems only where they too espouse syllabism as their principle, introduce phrasal accentuation, or rigorously observe the medial caesura, as in French." Common usage within the literatures of European languages is broader still, embracing lines syllabic, accentual-syllabic, and (inevitably) stationed ambivalently between the two; lines of 12, 13, or even 14 syllables; lines with obligatory, predominant, and optional caesurae.

Although alexandrines occurred in French verse as early as the 12th century, they were slightly looser rhythmically, and vied with the décasyllabe and octosyllabe for cultural prominence and use in various genres. "The alexandrine came into its own in the middle of the sixteenth century with the poets of the Pléiade and was firmly established in the seventeenth century." It became the preferred line for the prestigious genres of epic and tragedy. The structure of the classical French alexandrine is

Classical alexandrines are always rhymed, often in couplets alternating masculine rhymes and feminine rhymes, though other configurations (such as quatrains and sonnets) are also common.

Victor Hugo began the process of loosening the strict two-hemistich structure. While retaining the medial caesura, he often reduced it to a mere word-break, creating a three-part line (alexandrin ternaire) with this structure:

The Symbolists further weakened the classical structure, sometimes eliminating any or all of these caesurae. However, at no point did the newer line replace the older; rather, they were used concurrently, often in the same poem. This loosening process eventually led to vers libéré and finally to vers libre.