You write songs but you don't play bass. Worrying isn't it, because those low notes can make all the difference. Tony Bacon risks blood curdling vengeance by spilling some of the musical secrets carefully guarded by 4-string types.

LOOK, IF you're a bass player I can't actually stop you from reading this. Our secret agents cannot be everywhere.

But this article really is for non-bassists. Perhaps you're a keyboard player, a guitarist, or a drummer? Never mind, loves. If you're recording at home, or writing songs, or working out arrangements for the group, sooner or later you're going to have to come up with a bassline.

And of course that's simple, isn't it? Anyone can put a bassline to a song. True: anyone can add a lifeless, ordinary bassline to a song. But if you give it a little bit of thought and imagination and make it into something good, you can lift the whole number. For the bass is the most subversive instrumental part in rock. It can change the whole mood of a song simply by shifting a few notes away from the obvious. All you have to do is keep your thinking cap on.

Suppose we start by writing down the song's chord sequence? Relax. I don't mean notation, nor will the subject be mentioned again. Just scrawl down the names of the chords that the song uses.

The obvious bassline follows the root notes of those chords. When a Cmaj7 comes up, you'll play a C on the bass, an F#m and you'll play an F#, and so on. It's also obvious to place this root note on the beginning of each bar, or each chord change, and burble through the whole song like that. Definitely boring. A machine could do it.

So to get things moving around a bit, let's try to make the root note approach more interesting. First we can add extra notes. There is a good deal of theory attached to this sort of activity. Someone may have mentioned that you have to use particular notes under certain chords. Forget all that. What you need to do is experiment.

Try out lots of notes in addition to your root note — not only is this root something to bear in mind melodically, but it will help to think of this note as a sort of rhythmic root, too. Imagine that you're flying off from the root with other notes (which are totally up to you) and then coming back to the root every now and then. Try it a while, and remember or even write down the most successful notes. Success in these terms means the notes that work in the context of the song. These successful notes are the ones from which to construct your bassline.

Or try avoiding the root note altogether as you weave around. This is known as Harmonic Subversion. Actually I just made that up, but if you listen to what you can do with this technique, I think you'll agree that the bass is a powerful mood-changing tool. Try it.

Let's go off in yet another direction. What about trying to use just one bass note through a series of chords? Technical terms like 'pedal bass' loom, but we're only interested in the effect. Experiment again. Is there a single bass note that'll sit happily and harmonically under a passage of several chords? It can be so effective. If you can bear it, listen to Cliff Richard's 'We Don't Talk Anymore' as one of the classic examples of the potential drama of this scheme. You can get a lovely rich burpy bass sound going and fiddle around rhythmically, stabbing the note in when it feels good. Don't use this trick too often, though.

That single-note stuff does embody a quality that so many good basslines share: simplicity. The best bassists often convey the feeling that they're capable of being very flash, but aren't. And they use space as effectively as they use notes. Less is more; small is beautiful. Write it down somewhere.

Despite its jazz origins the 'riff' (essentially any repeated phrase) is one of the most useful rhythmic devices in rock, and its most apparent use is in basslines. Listen to great examples from Cream's seminal 'Sunshine Of Your Love' via James Brown's impossible 'Sex Machine' to Level 42's bass-and-synth 'Physical Presence'. So how do you make a riff stand out?

Well, you constrast it against something else. Listen to 'Physical Presence' again. That seductive, loping line on the verse evaporates when the chorus comes in, making the riff even more effective when it reappears. So make sure that a strong riff-based line in one part of your song is shackled to a looser approach elsewhere. Use your loaf: slice the song up into sections, and spread something different on each.

You can go usefully overboard on repetition: make a virtue of boredom, if you like. As Herbie Hancock did with 'Chameleon'. Or Frankie with 'Two Tribes'.

Another useful way of coming up with good basslines is to rid yourself of the feeling that you're merely lurching from chord-change to chord-change. Because an astonishingly liberating approach to the low notes is to aim to PLAY TUNES with your basslines. And we're not necessarily talking 'Birdland', either. Derek Forbes, the old Simple Minds bassist, once told me, "There's been a lot of our tracks where the bass has been a really major melody of the song," and you can hear what he means.

Your own tuneful basslines will straddle the chords of the song and coincide harmonically along the way with luck. But try to think in linear terms — one bassline will start at a certain point and lead to another point in a song (in other words it'll take up a chunk of the song's structure). On the journey between these points, you want in this instance to make the bassline as tunefully relevant as you can to whatever else is going on. Or not.

So where do we get the tune from? Most bassists will tell you that the biggest clues for good melodic basslines come from listening to the singer. If your tune has a vocal on it, take a good long cop of what the singer's up to. You don't want to nick the vocal tune totally. The trick is to absorb parts of it into your line, hinting at the singer's cavorting, while slipping in enough of your own notes to encourage the feeling of rhythmic motion.

And the best basslines do have this duality about them — they serve a melodic function to please the ears, and a rhythmic function to please the feet. You need listen no further than good old Tamla Motown for constant practical underpinning of such theories (my particular faves include the best of the Supremes, the Jackson Five and the Four Tops). The likes of Carol Kaye and James Jamerson played basslines of uncanny brilliance. Do listen.

Lastly, remember that you can play your bassline on anything you choose. Try it with a voice. Knock it out on a line of milk bottles with different amounts of water in them. Overdub bass synth with an acoustic guitar that follows the line exactly, and adds an edge to the sound (reggae uses this guitar-doubling-bass all over the place). And just in case those naughty bass players are still reading, we should mention that basslines sound jolly good on electric bass guitars.