Gettysburg Overview

Ranks

As a general rule of thumb, commanders rated the following ranks: 

Company: Captain 
Regiment: Colonel 
Brigade: Brigadier General 
Division: Major General 
Corps: Lieutenant General
Army: General 

In reality, the Union army was very stingy about passing out rank, especially 
Colonel and above. Until U.S. Grant came along, the highest anyone got was 
Major General.
At Gettysburg, many brigades were commanded by Colonels, and divisions often 
by Brigadiers. Other Divisions, all the Corps, and the Army itself, were 
commanded by Major Generals. Some Corps artillery brigades were commanded by 
Captains.

The Confederate army was better about rank. Most brigades were commanded by 
Brigadiers, and most divisions by Major Generals. There were exceptions, but 
not so many as in the Army of the Potomac. The Corps all had Lieutenant 
Generals, and Lee himself was a full General.