Scott Mattocks has wrapped up his series about LUCID development with the final letter of the acronym - D is for Documentation.
Despite mankind's best efforts, writing code is still clearly an exercise for talking to computers. It has not evolved to the point where talking to a computer is as easy and natural as talking to other people. That's why documentation is so important. Programming languages are just a translation of a developer's intent into something a computer can execute.
He points out that even a little documentation can go a long way (even in presentations with code in the slides). It provides context and the intent of the code, not just details about what it's doing. He proposes a compliment practice to test-driven development (TDD) that turns the documentation process around - Documentation Driven Development. This is essentially writing up what the code does first, then writing tests to check it and only then writing the code to make it happen.
If you're interested in the rest of the articles in the series, check out the LUCID article on his site with links to each letter's article.