Understanding the natural progression of scales in contemporary Jazz and Classical styles is a key to song writing and can also be used in practice.

This image of the circle of fifths / cycle of fourths is reversed to most you will see. The Cycle of Fourths runs clockwise and the Cycle of Fifths runs anti-clockwise. It still all means the same this and is created in the same way. Notice as we cycle through the fourths that a flat is added on that side of the image. When practicing or writing in a contemporary or Jazz style it can be handy to cycle your chord progression around in the fashion of the cycle of fourths. Alternatively, you can go the other way for a more classical style. How many songs have been written with a chord progression that goes C - G - D and then the occasional A. You’re probably doing it and don’t even know why.

When you circle through the fifths what you are doing is starting the next Major Triad or scale on the Fifth increment of the previous. If you want a Jazz feel go the other way and play the fourth increment.

An easy way to remember the cycle or the circle is to know that the keys (without incidentals) are C at the top, F then BEAD G BEAD G in the direction of the cycle of fourths. The first BEAD G is all flat and the second is all natural.

Modes are produced by displacing the starting point of the scale without changing it’s the interval formula.
What this means is that depending on where you start a scale depends on it’s mode. During the process the notes of the original scale stay the same. It’s alway simple to use the C Major scale to give examples. It has no Sharps or Flats (accidentals).

The first model name we’ll look at is the IONIAN. This is the natural Major scale
C-D-E-F-G-A-B

Now, for example, If this scale is started on the D then the interval patten will change and give a different ‘flavour’. It is still a C Major pattern but it now starts on the second degree. Starting on the second degree will create D DORIAN.
D-E-F-G-A-B-C

Here is a complete list of MODES as taken from ‘The Original Guitar Case Scale Book’ By Peter Pickow

The seventh mode, Locrian, was avoided for centuries due to it’s truly weird flavor. The scale outlines a diminished chord, melodies written in Locrian never seem to come to rest. This is commonly used in jazz for soloing over half diminished chords.

As a bass player you’ll find that you may not use every note in these scales. One needs to be wary that their phrasing doesn’t create the tonal qualities of a chord other then the one that is suppose to be being outline. I recommend alway spending as much time as possible working around the notes of the chord that is being played, or that is trying to be expressed. Tonic notes will almost always produce consistency with a song or piece of music.

As guitarist, there is always some room for movement but in the same way a bass guitar will move around the tonic notes it is also advisable for a 6 string guitarist or pianist etc. to consider the same.