No no, for the tonic, you can double the root or the 5th, never the 3rd. I just used C as an example if it was the tonic. If A minor was the submediant for the key of C, then you may have the option of doubling the 3rd. Many more rules, etc.
Appoggiatura
Take one bar, 2/4 time signature, in the key of C major.
Soprano = high C
Alto = E
(these two notes are above middle C)
Tenor = G (in bass clef)
All three notes are half notes.
Bass: D (quarter note) moving to C (also a quarter note)
That D is an appogigatura. It doesn't belong to the C major chord. It is on the accented strong beat, and is temporary, being displaced by the C.
Pedal Point
Keep the bass as C.
Soprano, Alto, and Tenor may alternate between C and G chords. So, the C chord matches the C in the bass, but the G chord doesn't match the C in the bass, yet it still sounds okay. That C in the bass is the pedal point, because it's like the note C is being held by the pedal, figuratively speaking.
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