Chord builder · treble staff · no key signature
A chord is three or four notes played together, each with a job: 1, 3, 5, and sometimes a 7. The job fixes the letter — the 3 of C is always some kind of E — and the sound decides which kind: E, E♠or E♯.
Each white key lines up with one place on the staff. Pick a note and a kind of chord; click any key or notehead to hear it.
Start on your note's letter, skip one, take the next, skip one, take the next: C · D · E · F · G. Do that and the three noteheads stack up neatly on the staff — all three on lines, or all three in spaces. If the middle note ends up right next door to the bottom one, the spelling is wrong, even if it sounds fine.
Play nothing but white keys and you already have seven chords. You don't pick whether they come out major or minor; that depends on where the small gaps in the scale happen to fall. Click any of them to load it into the workbench.
Once you know the letters, there are only two numbers left to sort out: how many keys up to the third, and how many up to the fifth. Count every key you pass, black ones included. (One key is what musicians call a semitone.)
Every chord here is a small third and a big third stacked on top of each other. Put the big one underneath and it's major; swap them round and it's minor. Use two small ones and you get diminished; two big ones and you get augmented.
You can keep going. Skip one more letter and add a fourth note on top and you've got a seventh chord — C E G B is CMaj7, and dropping that B by one key to B♠makes it C7, the chord that turns up in nearly every blues song ever written.
On the keyboard they're the same black key, and they sound identical. On the staff they're two different notes, and only one of them can do the job of the third.
Whole chords have the same problem. C♯ major and D♠major are the same three keys, but one takes three sharps to write and the other takes two flats — the Same sound, spelled another way buttons in the workbench flip between them so you can see the difference.
C to E skips a letter, so the middle notehead sits two steps up. The flat drops the sound by one key but doesn't move the notehead. The three notes still stack evenly.
D is the letter right next door, so the notehead only moves one step. The stack is lopsided now, and whoever's reading it has to stop and work out what you meant.