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Hello @tonyf60
I was not going to reply once I read @Loraine ‘s comment. She gave the precise answer. Dominant is a term from Functional Harmony. All the 7 notes, or more specifically, the 7 degrees (the positions within the octave, what you know as “the matrix”), are given names in Functional Harmony. The only ones you hear regularly are the dominant, or 5th degree, and the tonic, the 1st degree and the octave, the 8th degree.
Normally, no one calls the 5th triad the dominant chord, but it is. You will usually only hear the name “dominant” when the 4th note is added. The reason for the special name, dominant 7th, is because it is the only major chord found in a diatonic scale, or your matrix, that has a minor 7th.
What @SoCal_Ian shared, is the “formula” for the dominant 7th. Formulas are derived from the tonic, or root, of the major scale. So, if you look at the root of the major scale, you have a major triad with a major 7th as it’s 4th note. So, the “formula” for the dominant 7th is to flat that 4th note, the “7th”. Thus, it’s formula is 1 3 5 b7.
Hopefully now, you can reconcile the direct explanation Loraine gave with the extra info Ian gave. Both are valuable as you will hear people refer to the “flat 7”, but you will know exactly where that “flat 7” comes from and thus, why it’s a minor 7th instead of a major 7th.
Ok, so the “simple” answer to your question, it’s the 5th degree, or 5th note in the chord matrix.
MG 🙂