Was doing some thinking on what makes an architecture truly reusable

One of the things that I have been pondering is how. I was really pondering this because I was working on something this weekend that is to be truly reusable, and one of the things I kept thinking about over and over is:

• understandability

That might not be a real world, but the key point I am trying to make is that no matter what you do, or what you work on - if you want it to be used by someone else or thousands of someone elses, you need to insure that it is actually understandable; otherwise it cannot be replicated. 

So - how? How do we make things understandable?

That, of course, is a tougher question. It's also harder, of course, to retrofit something existing than to create something from scratch.