The Senior Captain rank, which was originally called Captain of the Line, was a position within the Imperial Navy. It was one of two line ranks that together formed the overall position of captain. It outranked the position of Senior Commander, but was a lower rank than Rear Admiral. This rank was the equivalent of the rank of Colonel in the Imperial Army and the Starfighter Corps. A Captain of the Line was the commanding officer of a tactical unit known as a line of battle. While lines were intended to be a unit of four starships, in practice, they could range from a single ship (a single Star Destroyer was considered a line in itself according to the Navy Order of Battle) to as many as twenty ships. Senior Captains were given the title of Captain when they commanded only one starship, but held the title of Commander when they commanded a small group of escort ships, below the squadron level. Captains of the Line often served as an adjutant to an admiral.
Thrawn held this rank not long after the Battle of Yavin in 0 ABY, before being promoted to flag rank. He held it in the months following the destruction of the first Death Star's while on a mission against Nuso Esva.
The Captain of the Line rank first appeared in the Imperial Sourcebook that was published in 1989. When Timothy Zahn introduced the Senior Captain rank in his 2011 novel Choices of One, which occurred during the writing of the 2012 reference book The Essential Guide to Warfare, Jason Fry had to revise his Imperial Navy Rank Guide to include it. Eventually, the ranks of Senior Captain and Captain of the Line were combined and treated as the same rank.