Canonical posts are designed to answer common questions in depth. They can be used as duplicate targets when appropriate and also used as references within other answers.
A canonical post can be created from a regular question, or it can be deliberately created and then self-answered. Answers may be posted by multiple people or they can be Community Wiki answers.
A canonical provides detailed complete answers for commonly asked questions ("duplicate"), general information such as tools and tips needed for a variety of questions where the particular question may not require all of the information included in the canonical post but where it will be easy to find and comprehensive.
