The third brightest of all modes is the Mixolydian mode. If you are not familiar with the ranking of modes from brightest to darkest, please read this before moving onto the Mixolydian mode. One very useful thing to remember is that we will study all modes based on the Ionian mode. The brightest of all modes Lydian not only has all the bright notes of Ionian but it has a raised fourth, which adds an extra layer of brightness to the overall tonality (read more about Lydian mode here). As we move from Lydian to Ionian, the tonality gets slightly darker and that comes from  flattening the fourth. So the idea here is that the more notes we will flatten, the darker it will sound. So on that same ground, moving from Ionian to Mixolydian, which is even darker, we can expect that a note is going to be flattened, and that for Mixolydian is the 7th note. So if key of C (Ionian) is C D E F G A B then key of Mixolydian is C D E F G A Bb.

The chords in C Mixolydian are the followings. Remember the number 357 as these are the chords that will be different from our chords in C Ionian.

C                                              C7

Dm                                        Dm7

Edim                                    Em7b5

F                                             FM7

Gm                                          Gm7

Am                                          Am7

Bb                                          BbM7

Along with the 3rd 5th and 7th chord, also notice that IM7 is actually a I7 chord in the key of C Mixolydian. All these 4 chords contain the flat 7th note which carry the Mixolydian tone so a chord progression with an emphasis on those chords will have a strong Mixolydian flavour.

The first part of Sweet Home Alabama by Lynyrd Skynyrd is written in D Mixolydian with an emphasis on the b7 chord.   Mixolydian is very popular in blues and also in rock and roll, AC/DC used Mixolydian extensively in their songwriting.