This video from youtube makes the answer simple – know these chords.

“I have been playing for a number of years and have done the private lesson
and the books and so on but could never find what I really needed/wanted,
spent hundreds of dollars.

I have found what I need and I wish to thank you, this is very good and as I
have played with some well known names in my time in the music business, I
will pass this info along to other folks who might wish to enroll in your
very well presented course.

I do not play professionally at this time ;( quieter life style at the
present) but I am excited to be able to tell folks where to go to get what
they need.” … Gary Hamilton, El Paso, TX.

I would love to hear your success story.

As the 1940’s came, the jump blues style characterized by big band music sequestered the guitar to the rhythm section primarily. The primary influence of this era on blues guitar is that it heavily influenced the development of what would later be known as rock and roll, or rhythm and blues.

After World War II in the 1950’s, blues guitar became electrified and amplified. Starting in Chicago, this new electric blues was characterized by the sounds of Howlin’ Wolf, Muddy Waters, and Jimmy Reed. All of these players grew up in Mississippi, but migrated to Chicago. The bands typically had, in addition to the electric guitar, harmonica and a rhythm section of bass and drums. Sometimes there would also be a saxophone, though it would be relegated to a rhythmic support role.

B.B. King and Freddie King were also making names for themselves at this time. They were somewhat unique at the time because they did not make use of the slide to play the guitar. B.B. King has long been considered one of the greatest blues guitar players of all time. Freddie King has often been called the King of the Boogie Woogie guitar.

While Chicago had it’s own sound in the 1950’s, some other artists such as T-Bone Walker and John Lee Hooker were creating what some call the California Blues style. T-Bone Walker was born in Dallas, while Hooker was born in Mississippi.  The California Blues Style that they helped to forge was smoother than the Chicago Blues and is somewhat of a melting pot for Chicago Blues, jump blues, and some jazz swing.

Starting in the 1960’s, Caucasian audiences gained more interest in blues guitar thanks in part to the Paul Butterfield Blues Band and what was later to be called the British Blues Movement. Bands such as Fleetwood Mac, Cream, John Mayall and the Bluesbreakers, the Rolling Stone, and the Yardbirds were performing classic blues tunes in addition to their original tunes. Many of these artists inspired American blues-rock artists like Janis Joplin, Jimmy Hendrix, and Johnny Winter.
Meanwhile, in Chicago, Albert King, Buddy Guy, and Luther Allison where creating what is called the West Side style of Chicago Blues. Their bands were dominated by the amplified electric blues guitar and heavily influenced later artists such as Stevie Ray Vaughan, Johnny Lang, and Kenny Wayne Shepherd.

Since the early 1980’s, the blues has enjoyed quite a resurgence in America. The Texas Rock-Blues Style of Stevie Ray Vaughan and The Fabulous Thunderbirds brought the blues to American rock radio stations. Eric Clapton, who originally gained his fame with Cream and John Mayall, continues to make great blues guitar albums and even recently recorded a set of old Robert Johnson classics. Many famous, legendary blues guitar players such as Buddy Guy and B.B. King continue to share the stage with the new generation of blues guitar players like Robert Cray, Joe Bonamassa, and Walter Trout.

All of the forms use only three chords, and roman numerals are used to describe them. The first chord, called the I chord, is usually a dominant seventh type of chord that is built from the first note in the major scale. If you are in the key of G, then G7 will be your I chord. The fourth note of a G major scale is a C, so the next chord used is called the IV chord and would be C7 in the key of G. The fifth note in the G major scale is D, so the V chord would be D7. So now you have your three primary chords, G7, C7, and D7.

The 12 bar blues is 12 measures long. The term bar is just another word for measure. The first bar is always the I chord. The second bar is either the I chord again, or for a ‘quick change’ blues, it is the IV chord. Bars 3 and 4 are always the I chord again. Bars 5 and 6 will be the IV chord, and bars 7 and 8 will be the I chord. Bar 9 is the V chord, but bar 10 will either be IV or V. Bar 11 is the I chord, and Bar 12 is the V chord which sets up the listener for the start of the next chorus.
The 8 bar blues, as in the song “Key To The Highway,” also starts with a I chord. After that, Bar 2 is the V chord. Bars 3 and 4 belong to the IV chord. Bar 5 is back to the I chord. Bar 6 is the V. Bars 7 and 8 repeat that sentiment with the I chord on Bar 7 and the V chord on bar 8. Once again, the final bar has the V chord to setup for the next chorus.

A 16 bar blues, such as the Herbie Hancock tune, “Watermelon Man,” starts like a 12 bar blues. The first four bars are the I chord. Bars 5 and 6 the IV chord, and bars 7 and 8 return to the I chord. Even bars 9 and 10 share the V and IV chord usage like the 12 bar blues. The difference is in that bars 9 and 10 are repeated 2 more times for Bars 11 – 14. Finally, bar 15 returns to the I chord which also holds on for bar 16.

As a blues guitar player, remember that it’s not that important how you play the chords in a blues tune. You can play the same tune with several different comping styles and get several great results for the same tune. Similarly, when you find a good comping pattern that you like, don’t be afraid to stick with it for a lot of the tunes that you play. If it works well in one tune, chances are it will work well in others.