Where did the blues first develop?

African American communities in the southern USA

What is the usual chord pattern of the 12-bar blues?

I, I, I, I, IV, IV, I, I, V, IV, I, I

AAB: a line, the same line again, then an answering line

The singer sings a line and the guitar or harmonica answers

Pairs of notes played long-short instead of evenly

Electric guitar, bass, drum kit and piano in a band

By sliding a glass or metal tube along the strings

Blues grew up in African American communities in the southern USA, especially the Mississippi Delta, in the late 1800s and early 1900s.
It came from work songs, field hollers and spirituals, and it was passed on by ear rather than written down.
AQA studies blues from 1920 to 1950: from solo Delta blues to electric Chicago blues.

The 12-bar blues is a chord pattern that repeats: four bars of chord I, two of IV, two of I, then V, IV, I, I.
Blue notes are the flattened third, fifth and seventh. They give blues its sad, bending sound. The blues scale is 1, ♭3, 4, ♭5, 5, ♭7.
Lyrics often follow an AAB pattern: a line is sung, repeated, then answered by a different line.
Call and response: the singer sings a line, and the guitar or harmonica answers with a short fill.
Rhythms often swing or shuffle (long-short pairs of notes) over a steady 4/4 beat.

Delta blues (1920s and 1930s): one singer with an acoustic guitar, sometimes played with a slide (bottleneck guitar), and harmonica. The rhythm is free and the feel is raw.
Chicago blues (1940s and 1950s): musicians who moved north added electric guitar, amplified harmonica, piano, bass and drum kit, playing as a band.
Other features: walking bass (a bass line moving in steps on every beat), improvised solos, boogie-woogie piano.
Blues led to rhythm and blues, rock and roll and rock, and influenced jazz.

12-bar blues: A repeating 12-bar chord pattern using chords I, IV and V.
Blue note: A flattened third, fifth or seventh, bent for expression.
AAB lyrics: A line sung twice, then answered by a different line.
Call and response: A phrase answered by another phrase, often voice then guitar.
Swing rhythm: Pairs of notes played long-short instead of evenly.
Walking bass: A bass line moving step by step on every beat.
Bottleneck guitar: Guitar played by sliding a glass or metal tube along the strings.
Delta blues: Early acoustic blues from the Mississippi Delta.

Where did the blues first develop?: African American communities in the southern USA: Especially the Mississippi Delta.
What is the usual chord pattern of the 12-bar blues?: I, I, I, I, IV, IV, I, I, V, IV, I, I: Three groups of four bars.
Which notes are flattened to make blue notes?: The third, fifth and seventh: They are often bent or slid into.
What is the pattern of most blues lyrics?: AAB: a line, the same line again, then an answering line: The answer line often rhymes with the first.
In blues, what does call and response usually mean?: The singer sings a line and the guitar or harmonica answers: The instrument's answer is called a fill.
What is a swing rhythm?: Pairs of notes played long-short instead of evenly: It gives the shuffle feel.
Which instruments did Chicago blues add to the Delta sound?: Electric guitar, bass, drum kit and piano in a band: Musicians moved north and plugged in.
How is a bottleneck guitar played?: By sliding a glass or metal tube along the strings: It makes sliding, wailing notes.