Tchaikovsky in his "Guide to Harmony" states the following about the dominant ninth chord (G-B-D-F-A) (translating from the original language):
"Being strongly dissonant, the dominant ninth chord requires a preparation, i.e. the ninth interval (A) should be present in the previous chord in the same voice. Triads on 2, 4, 6th step can be considered as a preparation".
Now, from the previous reading the dissonant chords are "prepared" by supporting consonant chords which have the same tones in the same voices.
Question: Is there any particular reason that we need to prepare specifically the NINTH (A), and not e.g. third, seventh?...
P.S. Yes, I know that the tonic triad can be used before this chord and it does not have common tones.