NB Essential to playing jazz and fully understanding some examples in this will be the ability to read bass clef.
One of the major things you need to know for walking is how to construct scales and chords. Scales and modes here. Chord Construction here & there.
Now, first thing, a walking bass line is a line in which the bass player lays down a chord progression and a tempo. The ultimate goal of a walking line is to combine Harmony, Rhythm, and to an extent Melody.
For the most part a walking line you will create will be in the standard 4/4 swing style so for a rhythmic line you will more than likely be playing quarter notes or the odd eighth note. Your job is to lay down the time so don't go playing some syncopated sixteenth rhythms or you might get some looks from the tenor player up in front of you trying to solo or an odd kick from the drummer. Speaking of the drummer you could help him along with the rhythm and slighty accent beats 2 and 4 of the bar. Slightly. Only do this in every bar if playing big band swing, otherwise it sounds rather cheesy in small bebop groups. Just throw it in once in a while if it seems like the piece is starting to drag or sounding too straight.
The harmonic portion of your line is where some more complications come in. Almost always when playing a walking bass line you want to be playing the root note on any chord change. Actually scratch that, always (unless there is a more sensible smaller leap see example 1, however it should be a chord tone) when there is a chord change play the root note. Now, you may ask, what if the chord stretches over two or more bars. This is where *some* leniencies can be made. If you find yourself on the second bar of the chord and you don't want to play the root you at least want to play a chord tone so for example you're on a C7 chord for two bars on the first bar you first note must be a root but on the second bar it could be maybe an E a G or a Bb. (see example 2) This rule can be overruled which I will talk about later. Now you know what note to start on but what about the other three notes in the bar? Well this is where some creativity can come into play. The second and fourth notes of the bar are known as the weaker harmonic beats, or the stronger rythmic beats.
For the second note you can play virtually any note with a few exceptions. In general, you want to play something scalar or tonic but even so you can play something else. An exception to what you could play would be the minor second of the chord *but* again the same overruling that takes care of the chord tone rule takes care of this. The minor second is just an ugly note and should be avoided like the plague without the ultimate rule. A good thing to try and strive for is a leading tone of some kind to the next beat of your bar like a Upper Chromatic Neighbour Tone or a Lower Chromatic Neighbour Tone or any other kind of Neighbouring Tone.
Now for you third beat you're going to need a chord tone. This beat and the first beat are known as the stronger harmonic beats. Few exceptions to this rule. Even the ultimate overruling rule does not usually overrule this rule So with this in mind choose your second note carefully you need somehow reach a chord tone on your third beat, always keep this in mind.
For the fourth beat you want to play some kind of leading note into the next chord. This is probably your freest beat to play virtually anything that will get you to the next chord tone. Still preferably tonic but even less so than the second beat. When I say play anything I literally mean play any note you need to lead into the next chord, now obviously this is a slight exaggeration but your ear will more than likely tell you if you've made too big of a jump or some other kind of blip in the smoothness of the line.
If you are ever lacking in confidence for whatevr reason the safest walking line is always arpeggios but doing this all the time does make the line rather limited.
Now the melody is really the simplest and shortest part of your bass line. If you see in the melody any kind of odd note you should more than likely cater to that note. That's it basically for the melodic portion of your line.
The Ultimate Rule that Overrules Virtually Any Other Rule in Creating Bass Lines in Jazz.
Linear, logical, and chromatic motion of any kind overrules any kind of rule involved in writing a Jazz Bass Line. Seriously, if you're making a line, chromaticism is your best friend in the world. Anything concerning chord tones and harmonic beats and rhythm and anything else can all be substituted for good old' fashioned step-wise motion. It is the ultimate, ultimate override button in all of Walking Bass Line-dom.
Some Other Rules To Remember.
The fourth (in a major chord), along with the minor second, of the chord is considered dissonant and should be treated as such. Can anyone tell me why? No? It's because the fourth is a minor second above a chord tone (the third). Although it is reasonable to play these notes if they are chord tones such as in _11 or _7b9 chords.
A large jump of any kind should usually be followed by some kind of reverse in motion.
Probably the most important rule in all of making bass lines jazz or otherwise, let your ear guide you. If you listen to your bass line and something sounds funky, more than likely something in there is wrong so change it. Although sometimes a little dissonance goes a long way and can transform a piece.
Ballads are your slower jazz pieces and in general you have much more free reign in terms of both harmony and rhythm. I find for slower pieces that it helps to stay with chord tones for the beats in the abr but you can put in little chromatic runs as needed. Hopefully I'll record a slower version of Autumn Leaves showing you the type of ideas to use in ballads. In slower pieces it's often a good idea to play with a two feel for the majority of the time and add triplet fills every now and again. Don't make it too busy the first time on the melody or during solos. You don't want to draw attention away from the soloist, it's your job to be in the background supporting.
The typical latin bass line goes dotted crochet (quarter note), quaver (eighth note) on the root and fifth of the chord. Again use little chromatic runs or end on a chord tone which is a semitone away from a chord tone in the next chord. Latin is probably the easiest to improvise because it usually has a much more rigid harmonic and rhythmic structure
More Advanced Ideas
Pedal notes are sometimes given to you in the lead sheets when you have a long series of slash chords where the bass note is the same for long periods, for example Dolphin Dance by Herbie Hancock. However in other cases it is not so obvious. For example in Autumn Leaves it would be possible to play a dominant pedal (ie a pedal on the fifth of the key) in the first section. The chords go A-7 D7 Gmaj7 Cmaj7 F#-7b5 B7 E-. A pedal on the note of B (the dominant note in E minor) would work over all these chords apart from the F#-7b5 because the semitone gap between b5 and the p4 that the B is too dissonant, although you may want that dissonance in the performance to make a point in the piece. The best time to use the pedal is either in the intro before going into a full blown walking line. Another possibility is to use the pedal in the start of a solo and allow the soloist pretty much free reign with the scales they decide to use. A neat idea when playing prolonged sections of the pedal is to give the piece a 3 feel (see example 3). If you do this make sure the drummer knows otherwise it could get very messy.
All this means is that the chord progression of the chord of the second note in the scale, followed by the chord of the fifth note, followed by the chord of the root. This pattern in major keys for the root is _-7, _7, _maj7. For example in the key of C it would be Dm7 G7 Cmaj7. This chord progression sounds good because a V-I movement between chords resolves well. The ii acts as a V of the actual V in the key so D is the fifth of the chord; this makes the resolution extra good. In minor keys it is _-7b5, _-7, _-7. However the v (minor chord of the fifth note) is often made major because E7 -> Am7 sounds better than Em7 -> Am7. If you see a II-V-I pattern it means you can use any notes from the scale of the I in the II-V-I progression. EG you could use an E major scale to walk from through F#-7, B7, Emaj7. When using the minor scale be careful of whether it is v (minor) or V (major). To decide which minor scale to use: if it is a v chord use the natural minor scale. If it is a V chord use the natural minor scale however when walking over the V use the harmonic or melodic minor scales. This is because of the sharpened 3rd in the chord. (See example 4).
To spot a II-V-I cadence takes some practice but a tip is that if you get repeated intervals of a fourth up you've got a II-V-I, you just then have to identify the key. For example D->G is a fourth, G->C is a fourth. Once you've learnt to spot this cadence it will become your best friend because it is use so often in jazz, it's unbelievable. Even funkier jazz numbers such as Red Clay by Freddie Hubbard make use of this cadence.
Chromatic chord movement
Sometimes you may see a chord sequence such as Dm7, C#m7b5, Cmaj7. Well you've got the ii and I there but what's happened to the V? A normal jazz substitution is the tritone substitution. This is usually to provide the chromatic movement you see here. If you find the tritone of C# you'll find that it is G. So basically you've got a ii-V-I pattern, so you can use the rules stated above, however once on the fifth remember that it is C# instead of C. Normally the best thing to do with tritone subs is to just play the root notes for the first two chords and then walk for the I.
Other chromatic movements such which aren't from tritone subs are often best dealt with by just root notes or a fixed pattern like R1 V1, V2 R2. (See example 5)
Walking in three
Walking in a piece that's in three often is a different ketlle of fish. There are three ways of usually attacking this:
a) One note per bar
b) A note per beat
c) Minim then crochet (Half-note then quarter-note)
I usually use a mixture of b) and c) because that usually achieves the best effect. However there are cases when the one note per bar is definitely what sounds the best. Again, use the guidelines stated above to find the notes to play. For the example I'm using Someday My Prince Will Come for the chords. Another idea for pieces in 3 is to play as if in 4, see the last 2 bars to see how this works. (See example 6 for jazz waltz)
The two feel is in my opinion one of the best pieces in the walking bass armoury. You'll find that a lot of the work from the Bill Evans Trio uses a two feel during the head (or tune) with the bass occasionally doing a bar of walking every now and again. But once it comes to the solos, it's bars of four notes per bar. (See example 7)
This again is like the emphasis of 2nd and 4th beats of the bar, something to be used sparingly otherwise it sounds too much. You normally play a dead note before note in the rhythm shown in example 8. The semiquaver then dotted quaver rhythm produces the same kind of effect as the drummers hit of the snare or ride cymbal, often just to keep the piece swinging.
A diminished chord tip
If you have a major chord, followed by a diminished chord with it's root up a semitone you can use this little trick as shown in illustrated in example 9.
Please note this is just one approach to improvisation, there are hundreds of different ways to attack it, don't take this as the definitive approach.
Soloing is much like building a walking line, if you want to play it safe stick to chord tones and common extensions to those chords. However it can be much more than this. Generally when the bass solos in jazz there is little harmony accompaniment, at least in generally in small combo situations, like a piano trio or a quartet. And generally you'll have pretty free reign on the rhythm as well, because the drummer doesn't necessarily carry a strong and obvious beat. The first place in selecting the correct scales to play is to look at the key signature, a list of which can be found here. After looking at the key signature it's necessary to determine whether it is the minor or the major key. The best way is to see which chords the piece resolves to most and look for what would be the V in a minor key, if it is major it will generally be a minor key, if the V is minor it's generally a major key although there are obviously exceptions. Next have a look at the chord sequences, because in some pieces the key signature won't help you at for example giant Steps by John Coltrane. In Giant Steps the key modulates (changes) by going up a major third. The way to decide which scales to play is to look for II-V-I patterns much like in creating a walking line and use the scale of the I to solo for the length of that progression. Also note that in jazz that it's quite common to get incomplete II-V-I patterns, ie just the II-V but you can still play the scale of the I over this because it is still in the correct key. A further soloing idea is to build a collection of little licks based on the II-V-I pattern be it from jazz standard melodies or from transcribed solos. Having a collection of licks for this pattern is invaluable because the pattern is so frequent. It also gives you a collection of ideas on which to build the solo from and more time to come up with original solo ideas if it's the first time you've seen a piece. A final idea for soloing is to quote sections of the melody when you reach that relevant section in the piece. Then use that as a base to further your improvisation. The problem with this is that most lead sheets are written with the melody in the treble clef so you need to be proficient at reading the treble clef and then transferring that onto bass. One more thought is to leave gaps in the solo. You don't need to be constantly playing for it to be an impressive solo. I read in an interview with Marcus Miller that one of the problems he thought many young bassists had when soloing was that they tried to play too much too quickly and all the time, and that one advantage wind instruments had was that they have to leave gaps (unless they have perfected the art of circular breathing). To try and create this same effect he suggested to sing with your solo, this way you improve both your ear and singing abilities but you also have to leave gaps.
Here's a great example of a jazz solo played on Autumn Leaves. Please note this is just one approach to improvisation, there are hundreds of different ways to attack it, don't take this as the definitive approach.
Playing jazz on an electric vs an upright does require a completely different tack. One is that your mental angle has to think "upright and the sound that it produces. This will help you lay back a bit. Your attack is going to be slower and thinking about the physicality of an upright helps. Pluck closer to the neck and learn to mute slightly with your palm, For me, on the amp, I tend to boost the low ends and cut the mids and high end to get a really thick sound.
I've got audio versions of all the numbered examples and just did a quick go at a walking line on Autumn Leaves. It's not the same as the notation version but it has similar ideas.
Full piece walking lines
Green Dolphin Street: