Blog XDoc stands for eXecutable Document. Copy Developers spend most of their time figuring the system out I am often asked what I mean when I say that developers spend most of their time figuring the system out. Let’s unpack the statement. The oldest reference on the topic I know of dates back to 1979 in a book by Zelkowitz, Shaw, and Gannon entitled Principles of software engineering and design. It said that most of the development time was spent on maintenance (67%). Software development costs (1979) Granted, the book does not specify how the figure was obtained. Still, it was deemed an important enough problem to attract significant research attention since then.