The human ear is kind of gullible. Suggest something to it and it goes off and jumps to all kinds of conclusions. A perfect example of this is the chord. Now, a chord can be any old group of notes, but the most common number is three and these three fundamental notes are usually referred to as a "triad" (not the Chinese gang). We'll discuss these three little guys in a second. The real purpose of a chord is to provide your ear with a suggestion, from which it will infer a complete scale, key, everything. Just one triad will imply which melody notes will fit and which basslines will be satisfying or not. I'm sure that all of you have heard about how an iceberg is 90% below water, the remaining 10% being above water. A really good way to think about this is that the triad is the tip of an iceberg, suggesting all of the possible notes combinations that go with it, far above and beyond the three notes played. Pretty useful, huh? So how does one make a triad? Well, certainly not just by clanging three random notes at the same time, although that works from time to time. Let's examine why we need three notes. Not one, not two, but three. One note is not really an option, since any note played at random can suggest any scale, chord, key, in the universe. Oftentimes, people will play two notes, let's say a C and a G. These will sound good together, but they won't actually suggest a chord. Why? Because it is the third note, the one between these two, that is the defining characteristic. The third note, between the C and the G, will spark all of the inferences in your mind, because it is what differs between the two chords. For a major chord, play a C-E-G. For a minor chord, play C-Eb-G. The difference is gigantic, even though they are only one half-step apart! You can see now why you need three notes. If you just play one or two, your ear says, So what? One last thing I want to stress before I go: triads are about intervals, meaning the distance between the notes, not the notes themselves. Remember this and you'll be able to use these ideas in any key on the keyboard, even the one with all the black keys!