Design is accomplished by periodically assembling the development team and brainstorming a design for the next several weeks worth of work. The design is traditionally documented on CRC cards, which are simple index cards with design information written on them. This design is then transcribed directly into the code, such that the code is the repository of the design. XP is strongly focused upon the quality of the code. This is appropriate, since the code is the product.
Many processes focus their quality efforts on work products at the expense of the code.
But XP directs the engineers to take great pains with the structure and clarity of the code. Combined with a good browser, this makes an excellent repository for design information. In summary, much of the software industry today is pursuing bigger and bigger processes in the hope that it will help them produce higher quality software, faster, and cheaper. I believe this hope to be vain. XP is a lightweight process that eliminates most of the work products created by other processes, and focuses directly upon the creation of high quality software, on time, in budget.