I generally use either a wiki page (if there is an agreed upon common wiki), or a short memo document. A wiki is nice because it's easy to include a URL into a

I generally use either a wiki page (if there is an agreed upon common wiki), or a short memo document. A wiki is nice because it's easy to include a URL into a commit. The diagramming tool is a minor consideration except it should be easily editable by someone coming along later. The important thing isn't the tool, but that there is a common, known place to look and collect the info for the team. It could be as simple as a shared folder as long as everyone agrees. In general, I write and try to encourage "tech memos" that are short of capital-D official, complete Documentation, and instead focus on capturing the high-level view of some scoped aspect of the total system. Memos that cover say a walk-thru of one type of operation through the entire system, how a single feature interacts with the larger system, or a tricky bit of analysis, logic, or assumptions on some key point; all tend to be popular reading. Whiteboard pics are often included just for convenience if needed too.