#1
So, I need help. I've been trying to take my skills to the next level and learn music theory, which has been going pretty well. I've gotten most of chord construction down, but I don't understand on ones such as A11 for example (1,3,5, b7, 9th, and 11th) I get why the 1st, 3rd, 5th and 11th are there. But why the b7th and 9th? And why a flat 7th?
Thank you
#2
Well really the way you're looking at this betrays that you're kind of looking at this the wrong way: the notes are there because that's the combination of notes that make that sound. The name comes from the notes, not the other way around.

If you wanted a chord that just had the 1, 3, 5 and 11 that would be an Aadd11. A chord name that has the root and then the name of the tone it goes to would, strictly, have all the chord tones up to that one. So an A11 has all the tones up to the 11th.

It's a b7 because it doesn't specify that it is either a major 7 chord or a minor chord. A natural 7 (so 1, 3, 5, 7, 9, 11) would be an Amaj11, also a minor chord would be an Amin11 (1, b3, 5, b7, 9, 11). A chord that specifies an extension (7, 9 11, 13) but not a tonality is a dominant chord. You can also get a minor/major7 chord (1, b3, 5, 7) but that is explicitly specified and quite rare anyway; they have a very distinctive and tense sound that you don't hear very much.

Again though, you must realise that the notes come first and then the chord so taking that example, it's a chord that contains the intervals 1, 3, 5, b7, 9, 11 and therefore has the name A11; it doesn't contain those notes because of its name.

Chord naming can get to be quite a tricky subject but it's very much worth knowing, don't be afraid to ask if you have more questions!
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#3
Convention dictates you use all the alternate notes up to the one notated in the chord- i.e. 11 will have the root, 3rd, 5th, 7th and 9th in addition to the 11th. a 9 chord will have the root, 3rd, 5th, and 7th in addition to the 9th.

it has a b7 because A11 is shorthand for being a dominant chord, i.e. it has a major 3rd but minor 7th.
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