Accentual-syllabic verse is an extension of accentual verse which fixes both the number of stresses and syllables within a line or stanza. Accentual-syllabic verse is highly regular and therefore easily scannable. Usually, either one metrical foot, or a specific pattern of metrical feet, is used throughout the entire poem; thus one can speak about a poem being in, for example, iambic pentameter. Poets naturally vary the rhythm of their lines, using devices such as inversion, elision, masculine and feminine endings, the caesura, using secondary stress, the addition of extra-metrical syllables, or the omission of syllables, the substitution of one foot for another.