This brief reference gives informal descriptions of most of the J primitives. Not every primitive is included and some idioms, examples and other resources have been added where appropriate. Since the presentation is brief and informal, it is not a replacement for the main J references: the J Introduction and Dictionary, the J User manual and the J Primer. However, since the material is informally organized by topic, this reference may be useful when considering which J features might be relevant to a given problem. Some users may also find it helps locate gaps in knowledge that can then be filled in with the main references.