The standard 12-bar blues is a I-IV-V chord progression most typically divided into three four-bar segments. Blues progressions are almost exclusively played in 4/4 time and dominated by the root (I Chord), with the IV and V chords providing that extra bit of flavor to keep things interesting.
In whatever key you are in, 12-bar blues uses the same basic sequence of I, IV, and V chords. It is most easily thought of as three 4-bar sections. the first 4, the middle 4, and the last 4 bars. The first 4 bars just use the I chord - I, I, I, I. The middle 4 bars go IV, IV, I, I.
Regarding the anatomy of an arch, what is the springing line? The springing line is an imaginary line between where the curves of the arch begin on each side
