To begin with learning scales, no need to memorize 20 of them. Try to practice the most important, the most used related to common jazz progressions. That's the secret.

Scales are important, but chord sequences are important too. The question you must ask yourself is: What are the most used chord sequences in jazz standards? Surely major II-V-I and minor II-V-I that implies minor, dominant and major chords related to minor, major and dominant scales.

Basically, it's simple to understand, over minor chords you will be playing minor scales, over major chords you will be playing major scales and over dominant chords you will be playing notes from dominant scales.

What Scales To Play Over a Major II-V-I Progression?

In a first time the Dorian mode will be a good choice to improvise over minor chords. This is the first mode of the major scale and surely the most used to practice minor improvisations over a II-V-I sequence.

The Mixolydian mode will be played over the V. This is just a basis. There are many other options to play over dominant chords, depending on whether you want to play altered or not.

The Ionian mode, also known as the major scale, will be used over major chords.

What Scales To Play Over a Minor II-V-I Progression?

Over the II of a minor II-V-I, we will be using the Locrian mode. This a basic choice among many others.

The half whole diminished scale or the altered scale sounds great over the V7.

The harmonic minor mode or the melodic minor modes are good options to play over the I chord.

To sum up, in a first time, the basic scales to know are : The Dorian mode, the Mixolydian mode, the Ionian mode, the Locrian mode, the altered scale, the half-whole diminished scale, the melodic minor scale and the harmonic minor scale. Making a total of eight scales to practice in all the ways.

How To Practice Scales?

There are many ways to practice scales. Generally, when you learn a scale for the first time, you play all the notes in the order, one by one, starting from the lowest root to the highest.

This is a first approach to hear the sound of a scale. However, this way you will never enrich and develop your playing. To simplify learning and understanding, all the following examples are based on the major scale (Ionian mode). Notice that you will have to apply these working tricks below to the eight scales set out above.

4 Directions

The "4 directions" technique consists of playing ascending and descending scales and mixing these two directions together. The first direction is to play the scales in the twelve keys in ascending movement (up & up). From the lowest to the highest note of the guitar.

The second direction is down and down, just play the twelve keys starting from the lowest to the highest note.

The two other directions are a mix of the two previous movements: Up & down and of course down and up.