# Letters to a Young Lady (Czerny)/Letter 9

LETTER IX.
CONTINUATION OF THOROUGH-BASS.
Dear Miss,
Each interval assists in the formation of some particular chord; and if, therefore, we go through all the intervals in this point of view, you will become tolerably well acquainted with all the chords which can be employed in music.
The perfect unison is no real interval; but two different parts are often obliged to meet on the very same note, by which means the unison is formed. Ex.
The × shews where the above interval occurs.
The seconds are all discords, and, like all other dissonant combinations, require in general to be prepared, as well as resolved.
Preparation occurs when we previously take a concord suitable to the purpose.
To the minor second there is required, to make it a chord in four parts, the perfect fourth and the minor sixth. Ex.
To the major second is required the perfect fourth and the major sixth.
To the diminished third, which is a harsh discord, belong the false fifth and the diminished seventh. Ex.
The minor and major third belong to the perfect common chord, as you know; and these chords we have already become acquainted with.
To the diminished fourth is to be added the doubled minor sixth.
To the superfluous fourth belong the major second and major sixth, or, in lieu of the second, occasionally the minor third.
To the diminished or false fifth is generally added the minor third and minor sixth. It then forms the chord of the sixth and fifth, which we already know.
To the superfluous fifth belong the major third and perfect octave.
The diminished sixth is accompanied by the minor third and diminished seventh.
To the major and minor sixth we usually add the major or minor third and the octave; and we already know this chord as the first inversion of the perfect common chord.
To the superfluous sixth belongs either the doubled major third, or the major third with the superfluous fourth. Instead of this latter interval, the perfect fifth may also be taken.
To the diminished seventh belong the minor third and the false fifth.
With the minor seventh we are already familiar.
To the major seventh belong the major second and the perfect fourth, to which, as a fifth part, the perfect fifth may also be added.
To the diminished octave are to be added the minor third and minor sixth.
The superfluous octave is a mere passing note, and it may be accompanied by the major third and perfect fifth.
The minor and major ninth require the major third and perfect fifth by way of accompaniment, to which may also be added the minor seventh.
And from this you may perceive the difference between the ninth and the second.
​Most of these chords have likewise their different positions; and in what manner these are formed, I have already explained in the preceding letters, in treating of the common chord and the chord of the seventh. But I must again repeat that these chords must be practised in all the keys, if you wish to derive any practical utility therefrom.
But enough on this subject. My view was only to give you a general idea of Harmony or Thorough-bass; and when you begin the study of it in a regular manner,—and I hear with pleasure that you are shortly about to do so, and that your worthy teacher has selected for this purpose the excellent Treatise on Harmony by Reicha,—all that I have hitherto said on the subject will serve to facilitate the acquirement of this science.