The Dictionary states:

a short metrical composition intended or adapted for singing but it hardly does it justice does it?

 

How about: 

A song is the combination of melody and words that unify to catch a moment’s    emotion - more specific perhaps?

 

But how does a song work? It works when the sense of the lyrics match the mood of the music. It is possible, and indeed does happen - the lyrical crimes against Chopin’s glorious piano melodies are too numerous to mention - that almost any lyric will work over a great tune. It is also true that great lyrics don’t necessarily need a great tune – but they do need to make sense together. 

Here’s the thing though – what kind of sense? They could be agreeable (sad song – sad lyric); they could contradict (cheerful song – despairing lyric): they could be ironic (upbeat mood – unhappy lyric): or antipathetic (hard rock song – gentle lyric). Generally there is agreement, and the subtle misappropriations are usually the province of the more remarkable songwriter – those that can set a mood before delivering a message that betrays it; a sting in the tail if you like. For the beginner it is better to start with trying to reflect the moods of both elements. 

Look at the song Comfortably Numb by Pink Floyd. Already famous for their extended soporific arrangements suggestive of a world weary unhesitatingly bleak outlook on life and the music business, here, in a nutshell they capture precisely what that feeling is: numbness. The music moves slowly, but not too slowly, the lyric talks of distance, so nothing too concrete, and the length of each section is extended by the slow rate of harmonic change. 

The first vocalist whispers in a baritone – barely able to speak – over minor key chords. Then Gilmour counters with a chorus in high tenor, the melody hanging over major key chords and a rippling string accompaniment, as he sings of painlessness and how the hopes and dreams of a child are gone. The central guitar solo uses chorus chords as if there is still a chance to hope, to dream, but the closing fade is unrelentingly bleak and minor key – the heavy tread of the beat almost somnambular, and of course, that is the point. 

At the other extreme could be Everlasting Love by Love Affair. It is unrelentingly upbeat. brass and drums and a superbly energetic motownesque bass line support a yearning vocal singing about the feeling of one who realises he is in that moment hopelessly and helplessly in love and nothing can stop this flow. But also there is a sense of uncertainty, of ambiguity. Even in this 3 minute pop song we cannot know that his love is reciprocated. We know he has walked away before, so is his earnestness true; will she have him back, is he standing there desperately trying to convince her? We don’t know, and nor should we, but the song tells its story in a handful of sentences that represent a powerful and effervescent reflection of the mixed up unstoppable feelings that hopelessness in love suggests. 

What of the classical song – a Schubert masterpiece – Die Forelle (The Trout). Here we have piano and vocal alone to convey the essence of the poet’s words. Reportedly Schubert would read the poem; wander round for a minute or two; then go to the piano and out came the song. The rippling arpeggios of the Trout - not too even and a little ‘agitato’ so the water ebbs and flows – the lyric concerns the catching of a fish, the vocal melody sung it a relatively straightforward rhythm as the calm angler tricks the unknowing poor trout in the obscure evening light. 

For the songwriter this marriage of lyric and music is the key component. And while we could choose a wide variety of possible lyrics for any given melody, somehow we know when the one we have is the right one, and, if we don’t, well, perhaps it isn’t and we need to rip it up and start again?