I play guitar, and am attempting to teach myself the piano. Which notes from a scale form the chord on the keyboard? Is there a common rule for this?
Any keys you like. A chord in just two notes played at the same time, although typically three or more is used. Now, most combinations will sound bad of course (at least for a certain value of bad, it's all subjective) so the question rather needs to be specified into which chord you mean.
The rules to form nice sounding chords are complicated, but it's the same rules as for guitars, but because of the difference between the instruments you would typically not do the notes in the same order, or at least you would have the notes repeated higher and lower.
So the C Major on a guitar would for example be "E C E G C E", while on the piano you would just take "C E G". These are both C Majors, although there are complicated naming systems to actually name the guitar chord something more complicated, as it has an extra low E.
Pianos are both easier and more difficult than guitars. There are no barre chords you can just move up and down. On the other hand you can move every chord up and down with just minor modifications in how you hold your hand, which are needed because the black keys aren't regular.
So there are less chords to learn, you don't have to learn G Major separately from C Major, but you will have to learn to transpose the chords, which comes with practice, I'm told. :-)
The first, third, and fifth. For example:
C major scale: C D E F G A B C
Open up the piano so you can see the strings. They are laid out just like the frets of a guitar.
A friend of mine actually learned to play the piano this way as a little kid. He studied the strings instead of the keyboard. He didn't know anything about music theory -- not even the names "major" and "minor" -- but he noticed that a certain kind of chord (what we call "major") always looked a certain way: a hammer, skip three, next hammer, skip two, next hammer. Another kind (what we call "minor") was upside-down: hammer, skip two, hammer, skip 3, hammer.
The most common chords used in western music are tertian chords. This means that they are built up in thirds from their underlying scale. Take the C Major scale:
C D E F G A B
Now, starting with C, move up in thirds (skipping a note each time)
C E G B
This gives you the notes of a "C Major 7th" chord. Why is it called a 7th chord? Because it contains the 7th note of the scale:
C D E F G A B 1 2 3 4 5 6 7 ^
Why is it called a "Major" chord? Because the 3rd (E) and the 7th (B) are both major intervals.
Pick a different mode (or scale) and you get a different chord. Triads are 3 note chords, built in the same way (in thirds from the root). A scale with a minor third (Eb is a minor third above C) will be a minor chord*. Here's some further reading on chord theory:
* It could be a diminished chord, but I am assuming diatonic harmony here rather than diminished.
If I am reading your question correctly, I believe you are referring to the type of chords besides just the chords.
The question as to the basic chords are answered by others already so I will just extrapolate.
My advise is to forget about scales if you want the fast track to getting chords. It will just like touching your nose the other way around. We can get back to the scale theory behind it later. In my profession, I strife to make learn as simole as possible for my 5000+ student in my 25 years of teaching. I hope this helps you.
Just think in terms of semitones (or half-steps in American Terms) and sharps and flats. With only 9 formulas, you can create at least 144 chords. There is absolutely nothing complicated with forming chords if you do it this way. Peel open your eyes and ears.
You already learnt that Chords are formed by pressing the Root note, next note 4 half steps up and then 3 half steps up. ie. C E and G in C chord. You can get every other major chords by using this R - 4 - 3 formula.
Now, to get the minor chord, just flatten the middle note (i.e. E flatted to E flat). So Cm chord = C Eb G. I do not want to use R - 3 - 4 because by doing so, you end up memorizing 12 formulas at least. I just want you to remember 9 for the time being)
For the suspended chord, just sharpen the middle note (i.e. E sharpened to F) Sharpen means raise the note 1 half step. So for Csus or Cs4 or Csus4 = C F G. You will get this is "McGyver's theme"
For the Diminished Fifth chord, just flatten the rightmost (fifth) note (i.e. G flatted to G flat). So Cm chord = C E Gb. It is most commonly used in m7-5 like in Cm7-5 which is C Eb Gb Bb. It will most commonly be used as Bm7-5 when playing songs in A minor key. Bm7-5 = B D F A. When playing Dm E7 Am progression, replace the Dm with Bm7-5 and you will get the an even more beautiful color to your song.
For the Augmented 5th chord, just sharpen the fifth, rightmost note (i.e. G sharpened to G#) So for CAug or C+5 or CAug5 = C E G+. You will find this is "Greatest Love of All" and "James Bond Theme"
Now for the 6th, 7th and Major 7th. Basically it means:
For 6th ADD a note 2 half steps from the 5th note. C6 will result in C E G A. You will find these sweet sounding chords prevalent in Hawaiian songs.
For 7th ADD a note 3 half steps from the 5th note. C7 will result in C E G Bb. These chords with "unfinished" feelings alway appear in pairs with root chords. I.e. G7 with C chord in C key; C7 with F Chord in F key; etc.
For Major7th ADD a note 4 half steps from the 5th note. CMaj7; CMaj;CM7 will result in C E G B. You will find these big broad sounding chords prevalent in Movie Themes to give you the big countryside feelings. You will also find it in songs with minor keys long root chord progressions like Feelings which goes Am - AmM7-Am7-Am6-Dm...
For Diminished Chords, this is a special exception. All the 4 notes are 3 half steps apart. ie. Cdim = C Eb Gb A. You will find these weird sounding chords prevalent in Hawaiian and Jazz songs as a passing chord ie. C C#dim Dm G7.... or C Ebdim Dm G7..... One of Bach's songs also uses this (I forgot the name) which is commonly used in Dracula Vampire movies. Try it out and see.
For all the above, you will see that all these adds to the color of sound and like an artist, these are your shades that will help you paint your masterpieces. How you use them will determine the quality and output of your music.
With the above, you can create 144 chords at least. Test it out as below
Major (try out all the 12 chords) Minor (try out all the 12 chords) 7th (try out all the 12 chords) m7th (try out all the 12 chords) m7-5 (try out all the 12 chords) dim (try out all the 12 chords) 6th (try out all the 12 chords) 7th (try out all the 12 chords) Maj7 (try out all the 12 chords) Dim (try out all the 12 chords) Aug5 (try out all the 12 chords) Sus4 (try out all the 12 chords)
With so much 144 colors, you should have enough to paint your masterpieces. Only after mastering these, we worry about the 9ths, 11ths, 13ths later.
Of course a Chord is any combination of notes played together so that's pretty broad.
Most piano players and composers approach piano chords from music where the chords are explicitely written out in sheet music notation. For people learning jazz, there are chord dictionaries and methods where most chords are listed in several notations.
What I will do is to suppose you want to play on piano songs you know on the guitar already.
Here is a way to see piano chords from a guitarist perspective
by putting fingers on the frets in a certain order, you add or substract semitones to the member of this arrangement.
Another thing you can do (it is fun but it takes time, it is a way to explore the keyboard) It is a bit cumbersome, it is a lot more practical to do that on paper if you know and can read sheet music notation. Or find a drawing of a piano keyboard and make diagrams of the chords that way: it is the equivalent of a tablature for the piano, but learning traditional music notation is a real asset:
You can do the same thing if you know sheetmusic notation : write the guitar chord in sheet music notation (you can start from a tablature applying the same principle than in the game with fingers on the piano or looking at your fingers if you know how to name a note you play on the guitar) then add or substract an octave to certain notes so that it fit in your left hand.