system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do chords in instruments sound good, but if you play random notes, it doesn't sound that great? What property of sound makes chords sound good?
gpt
Notes in a chord are in a very specific pattern. 

Let's take for example major chords. They are composed of the root note, the fifth, and the major third. What does fifth and third mean? For example if C is the root note (this is the case for the C chord), then the fifth would be the (count to 5... C, D, E, F, G) the G note, and the third would be E. (In this case it is the major third, while the minor third would be Eb. But let's not dig too deep)

Back to the pattern: fifths and major thirds have a very specific frequency. If the root note vibrates at X Hz (for example the "middle C" is 261.6Hz), then the fifth vibrates at 1.5X (for example G is 391.995Hz) or in other words their frequencies are at 2:3 ratio. For the major third the frequency is 1.25X and the ratio is 4:5 (E is 329.628Hz). 

Other "even" ratios will give you other "nice sounding" notes, e.g. 1:2 will get you the higher octave note, 5:6 will give you the minor third, etc

The more easily your brain can recognize the pattern between notes, the more familiar/pleasant the chord will sound. Uneven relationships between notes result in unpleasant/frightening experiences; this effect is used for example in alarms and horns to signal for danger.