A 12-bar blues is a form. Twelve bars, then it turns around and starts again. The Twelve-bar Blues page writes that form out.
The pattern uses three chord jobs: I, IV and V, printed there as I7, IV7 and V7. The bar counts on that page are 4, 2, 2, 1, 1, 1 and 1.
This Songbook does not tag a song page as blues. Some songs share the three chord jobs. They are not the same thing as the twelve-bar form.
Four bars of the I chord open the form. Then two bars of IV, then two bars back on I. That is the first eight.
The last four are the turn. One bar of V, one of IV, one of I, then a bar that leads home. The pattern page is where that last bar is written.
Count the bars out loud the first time. If you lose the count, you have left the form, even if the chords still sound fine.
The Metronome with chords makes the bar obvious. Set a slow tempo. One chord change is already written as a one-bar cell. Do not rush the one-bar cells.
Amazing Grace is G, C and D. Those are I, IV and V in G. The song is a hymn in three, not a twelve-bar form. Do not call the page a blues.
Camptown Races is C, F and G. Yankee Doodle is G, D7 and C. Oh Susanna is C, G7 and F. Same three jobs, different tunes.
Play the pattern, then play the hymn. Notice that the hymn does not spend four bars, then two, then two. The form is the point of the pattern.
If you want the seventh colour, read the pattern. The hymn pages that print D7 or G7 already show a seventh on that one chord. They still are not a twelve-bar chart.
C major, G major and F major are comfortable homes for I, IV and V. Move the pattern with the key chip. The bar counts stay.
Three-chord Song is the three-chord loop without the twelve-bar lengths. Use it to learn the three grips. Use the blues pattern to learn the lengths.
A room recording of a band will not match this chart bar for bar. This pattern is a practice form. It is not a claim about a record.
When the twelve bars hold together twice, stop. Another loop can wait. The form is learned when you can feel bar ten coming.