I've published quite a few posts on harmony. The reason is that harmony is, I think, the big problem for composers at this moment. Why? The period from the 12th century up until a little after 1900 saw a long and fairly consistent development of harmony. It started with the most basic fundamentals, simply learning how to combine two voices and what intervals were consonant and what dissonant. There was a long see-saw between the differing demands or possibilities of harmony (the vertical) and counterpoint (the horizontal) but there was, until some time in the 19th century, a continual development in the sense of the composer learning to handle more and more possibilities. Then, in the late 19th century the system simply became overloaded and ultimately broke down entirely as Schoenberg called for the "emancipation of the dissonance" which meant the end of harmony as true harmony depends on a relationship between consonance and dissonance. Well, so does counterpoint, actually. This situation did not last very long. For one thing, audiences fairly decisively rejected atonal music and for another, in the late 60s and early 70s a number of composers, the so-called 'minimalists', revived harmony. This was not, and still is not, entirely successful, though. Today, I'm going to try and show you the reason why.
First, here is an excerpt from a score by Steve Reich, the slow movement of Electric Counterpoint written in 1987: