The titles of journals are abbreviated. There are no periods in the abbreviation. A period comes after the abbreviation, delimiting it from the next field. The abbreviations are standardized. The standardization was formerly incomplete and internal to organizations such as NLM. It is now formalized at the supraorganizational level by documents including "Citing Medicine" at Appendix A: Abbreviations for Commonly Used English Words in Journal Titles, ANSI Z39.5, ISO 4: Information and documentation -- Rules for the abbreviation of title words and titles of publications, and the ISSN.org List of Title Word Abbreviations (LTWA).