A hymn tune is the melody of a musical composition to which a hymn text is sung. Musically speaking, a hymn is generally understood to have four-part (or more) harmony, a fast harmonic rhythm (chords change frequently), with or without refrain or chorus.
From the late sixteenth century in England and Scotland, when most people were not musically literate and learned melodies by rote, it was a common practice to sing a new text to a hymn tune the singers already knew which had a suitable meter and character.
There are many hymn tunes which might fit a particular hymn: a hymn in Long Metre might be sung to any hymn tune in Long Metre, but the tunes might be as different as those tunes that have been used for centuries with hymns such as Te lucis ante terminum, on one hand, and an arrangement of the calypso tune used with Jamaica Farewell, on the other.
Editors bring extensive knowledge of theology, poetry, and music to the process of compiling a new hymnal. They seek texts that are capable of communicating complex theological concepts to lay people, and they strive to partner those texts to tunes which are singable by the non-professional musicians of a congregation.
When editors choose a text for the planned collection, it may already be paired to a tune that supports its meaning, catches its spirit, and allows for congregational participation. This pairing may be used elsewhere, even ecumenically recognized, appearing in many other hymnals. However, if a hymn has been linked to a tune the editors think is not the best partner for it, they can arrange a new pairing. Partnerships of texts and tunes can give special attention to the interpretive opportunities in a text by providing artistic support of the message through its musical setting. Editors must consider whether the important words in the text fall on stressed notes, whether climax points in the ideas correspond with musical climaxes, and whether the tempo for the music matches the style of the text.