I never understood the (roman numeral)135 that is supposedly a basis for all chords. Just wanted to know how it works?
...thanks

All of music theory is based off the major scale system. There are 7 notes in the major scale. The roman numeral tells you wich one of the 7 notes the chord is rooted from. The upper or lower case roman numeral tells you wether the chord is major or minor.

ex; 2 5 1 in Cmaj = D- G C

Sorry, I answered your question wrong.
CHORD CONSTRUCTION works like this for basic triads.
The 1 is the root of the chord. (lets say were talking about a c maj chord here, the scale the chord is derived from would also be c major)
the 3 stands for the 3rd of the chord, or 3 notes above the root (wich in this case would be E)
the 5 stands for the 5th of the chord, or 5 notes above the root.(G)

Here we have a C major scale -


C D E F G A B

If you start at C and take the 1st, 3rd, and 5th notes, you get a C major triad (CEG). If you start at D and take the 1, 3, and 5, you get a D minor triad (DFA). And so forth.

This is how all chords are built. And you can get variations on the 135 format. For example, the dominant 7th chord is built on the fifth tone of the major scale. Start at G and count the 1st, 3rd, 5th, and 7th tones, and you get a G7 (GBDF).

Hope that helps!

1,3,5 refer to the notes that make up the basic structure of a chord known as "the major triad" which is derived from "the Major Scale." (It's called the major triad because that particular chord is only made up of 3 tones) The major triad is the building block for all major chords (or for all chords for that matter, but for simplicity's sake let's just deal with major chords). Use the C major scale as an example:

C, D, E, F, G, A, B, C
1 , 2, 3, 4, 5 , 6, 7, 1

The numbers 1,3,5 would refer to the tones C, E, and G in the above example. The C major triad would be comprised of the notes C, E, and G. C would be the 1st tone (or the root note) of our chord. The E would be the 3rd tone. the G would be the 5th tone. You may have come across chords such as, C7, C9, C11, C13, CMaj7, C6, etc... in the course of trying to play a piece of music. For now, just realize that all of the chords mentioned above (and many others) are built on the major triad. This means that all of those chords contain the 1st, 3rd, and 5th tones of the major scale. Using othe above example, all of the chords I mentioned contain C, E, and G.

C7 -- C, E, G, Bb

C9 -- C, E, G, Bb, D

C11 -- C, E, G, Bb, D, F

C13 -- C, E, G, Bb, D, F, A

CMaj7 -- C, E, G, B

C6 -- C, E, G, A