J. S. Bach. Organ Fugue, in A minor.
In the second bar of the major subject at (a) is seen a modulation to the subdominant, imitated at the same point of the answer; and in the third bar of the minor subject at (b) a modulation to the relative major, replied to by a similar modulation to the relative major (G major) of the dominant.
84. In order to understand what is meant by a tonal answer, we must remember that each of the old Ecclesiastical scales, out of which our modern scales were developed, had two "modes," one of which was a fourth below the other, but contained the same notes. If the scale was from final to final (or, as we should now say, from tonic to tonic), and the dominant was in the middle, the mode was said to be authentic; if, on the other hand, the scale was from dominant to dominant, with the final in the middle, the mode was called plagal. Each scale was divided into two unequal halves by the dominant or the final. Let us take, for example, the old Dorian mode—
![\layout { \context { \Voice \consists "Horizontal_bracket_engraver" } }
\header { tagline = ##f }
\new ChoirStaff << \override Score.TimeSignature #'stencil = ##f
\new Staff \relative d' { \time 8/1 d1^"Authentic"\startGroup e f g \[ a_"Dominant"\stopGroup b c d \] \bar "||" }
\new Staff \relative c, { \clef bass c1^"Plagal"\startGroup d e \[ f_"Final"\stopGroup g a b c \] } >>](http://upload.wikimedia.org/score/7/i/7i2x5wny2muymr1dfop4axxwc08vy1z/7i2x5wny.png)
The dominant in the authentic mode and the final in the plagal are marked in this example. It will be seen that the lower half of the authentic scale has the compass of a fifth, and the upper half the compass of a fourth; while the plagal scale has a fourth for the lower half, and a fifth for the upper.
85. The old rule for fugal answer was that a subject made in
