A citation key references some external material that is part of a bibliographical collection. Currently, Zettelstore implements this only partially, it is ""work in progress"". However, the syntax is: beginning with a left square bracket and followed by an at sign character (""''@''"", U+0040), a the citation key is given. The key is typically a sequence of letters and digits. If a comma character (""'',''"", U+002C) or a vertical bar character is given, the following is interpreted as [[inline elements|00001007040000]]. A right square bracket ends the text and the citation key element.