farming, devils, gambling, magic, floods and dry periods were less common in post-war blues. [ 15 ] The basic 12-bar lyric framework of a blues composition is reflected by a standard harmonic progression of 12 bars in a 4/4 time signature . The blues chords associated to a twelve-bar blues are typically a set of three different chords played over a 12-bar scheme. They are labeled by Roman numbers referring to the degrees of the progression. For instance, for a blues in the key of C, C is the tonic chord (I) and F is the subdominant (IV). The last chord is the dominant (V) turnaround , marking the transition to the beginning of the next progression. The lyrics generally end on the last beat of the tenth bar or the first beat of the 11th bar, and the final two bars are given to the instrumentalist as a break; the harmony of this two-bar break, the turnaround, can be extremely complex, sometimes consisting of single notes that defy analysis in terms of chords. Much of the time, some or all of these chords are played in the harmonic seventh (7th) form. The use of the harmonic seventh interval is characteristic of blues and is popularly called the "blues seven". [ 23 ] Blues seven chords add to the harmonic chord a note with a frequency in a 7:4 ratio to the fundamental note. At a 7:4 ratio, it is not close to any interval on the conventional Western diatonic