Poems that do not rhyme but still follow regular metrical patterns are called blank verse poems. Poems that do not rhyme or follow any metrical pattern are called free verse poems.
Blank verse can be written in any meter, but it is most commonly written in iambic pentameter. Iambic pentameter consists of lines with five iambs, or sets of two syllables with every second syllable stressed. One example of blank verse is Robert Frost's poem "Mending Wall," which is written in iambic pentameter except for its first line. Meanwhile, free verse has no restrictions; anything goes. Robert Frost once likened it to "playing tennis without a net." One famous example of free verse is the poem "The Red Wheelbarrow" by William Carlos Williams.