In songs, we often have very similar melodies and rhythm patterns being repeated over and over again. The following is an approximate melody of one stanza of Willy O' Winsbury as performed by Pentangle. (I prepared the score so it may be wrong -- I know very little about music.) This pattern is repeated throughout the song: 18 bars for a stanza, with a visible 5-4-5-4 pattern, corresponding to the four lines in a stanza. The melody is very similar every time the pattern is repeated, but it's not exactly the same. There are changes here and there to fit all the syllables in the song properly, to accentuate the proper syllables and simply to avoid making the song tedious.
I would like to learn to talk about such things.
I think there should be a question to which the answer is "18 bars for a stanza, with a visible 5-4-5-4 pattern, corresponding to the four lines in a stanza", a question like "What is X in Willy O' Winsbury?" I would like to know what the X is called, and how to formulate the answer to the question properly. For example, if I removed the fifth, the ninth, the fourteenth and the eighteenth bar, I would change a certain aspect of the structure of the song. I would like to know what the name of this aspect is.
Also, is there a name for the abstract "melody" that is repeated in the song? The actual melody is not repetitious. It almost is. Is there a general term for the class of all particular, very similar melodies corresponding to the stanzas? Is it a theme? It's not like in a classical or jazz piece where there's a piece of melody, called a theme, that pops up in various places, sometimes changed very much. Here it's almost unchangable (especially the X stays the same, but also the pitch of the succesive notes -- only some of them are sometimes split or merged), and there's nothing besides it.