One of the most important features of poetry is the meter which is the structure followed to construct each verse on a poem and that varies according to the number of syllables in each verse and whether these syllables are stressed or unstressed, this also means different combinations of stressed and unstressed syllables determine the metric of a poem. Additionally, the meter is measured by feet which are the units composed by one stressed syllable and at least one that is unstressed. In the case of English poetry the most common foot is the iamb which consists of the combination of one stressed syllable and one unstressed syllable, this is considered as the basic foot in poetry and has been widely used in poems in English; additionally, the preference for one metric or other also depends on the language and in the case of English the Iamb seems to fit with the language features. Thus, the most common foot in English poetry is the iamb.
The most common foot in English poetry is the "iamb" but dactyls are also fairly common. It really is up to the preference of the author to choose the poetic meter.
