In the English language hymns occur in a limited variety of poetic metres.
To put it more technically, such hymns have couplets with four iambic metrical feet in the first and third lines, and three in the second and fourth.
It is rare to find any significant metrical substitution in a well-written hymn; indeed, such variation usually indicates a poorly constructed text.
A hymn may be sung to any tune in the same metre, as long as the poetic foot (such as iambic, trochaic) also conforms.
Some of the most frequently encountered however are instead referred to by names: Two verses may be joined and sung to a tune of double the length: English minister and hymn writer Isaac Watts, who wrote hundreds of hymns and was instrumental in the widespread use of hymns in public worship in England, is credited with popularizing and formalizing these metres, which were based on English folk poems, particularly ballads.