Lieutenant Commander(abbrev: LCDR) is an officer paygrade within the Royal Manticoran Navy, the Grayson Space Navy, and the Republic of Haven Navy. It holds the equivalent military paygrade of O-4 within The Royal Manticoran Navy: The Official Honor Harrington Fan Association, Inc. and can be a division head on a ship or command of a destroyer. A Lieutenant Commander is always referred to as a "commander."
In peacetime, a Lieutenant Commander would have served three years as an Lieutenant (senior grade), between nine and eleven years in the service, and been reviewed by a Officer Promotion Board. Board review is more difficult, confirming only 80% of eligible candidates. 
If the spacer has completed their Lieutenant (senior grade) Exam, and served as an Lieutenant (junior grade) for twelve months Time in Grade, they are eligible to be promoted to this rank, but needs to confirmed by an Officer Promotion Board.
- O-4 equivalents from other branches of The Royal Manticoran Navy: The Official Honor Harrington Fan Association, Inc.: