Ranks during the Civil War - US Army, vs. brevet, vs. Volunteer.

During the American Civil War regular United States Army officers sometimes had up to four different ranks.
First they had their "permanent" rank in the regular army. In addition, however, they could also hold either
(or both) a rank in the volunteers or brevet rank (regular army and / or volunteers). Thus a single officer could
hold a permanent rank of major in the United States army, and at the same time hold the rank of colonel of
volunteers, a brevet rank of colonel in the army, and a brevet rank of brigadier general in the volunteers.

Now to sort that all out.

First - just what was a brevet rank?

Authorized in 1806, a brevet rank was usually an honorary promotion given to an officer or enlisted man for
gallent or meritorious service or conduct. A captain, with a brevet rank of lieutenant colonel could address
himself in correspondence as Lieutenant Colonel, but otherwise would not be given a command above a
company. Under most circumstances he would continue to wear the uniform of a captain, continue to draw
the pay of a captain, and could not give orders to majors, etc.

A exception to the above would occur in commands that were a mixture of regulars and volunteers or militia.
In such cases command went to the officer with the highest rank, be it regular or brevet. Thus you could have
a regular major, who was a brevet colonel be in charge of a brigade even though there were regular or volunteer
officers in the brigade with permanent ranks of major or lieutenant colonel. This was often done during the Civil
War (and earlier during the Mexican War) to allow presumably more experienced officers from the regular army
to be placed in command over less experienced volunteer or militia officers. While in such a position the brevet
officer was entitled to the pay and other privileges of the higher rank.

Many times a brevet rank would be for the benefit of the service, and thus temporary. A prime example of
such is George Armstrong Custer. Graduating from the U.S. Military Academy at the start of the Civil War
Custer was promoted to second Lieutenant. During the war Custer was promoted to first lieutenant and
captain in the regular army. He also received brevet promotions to brigadier general and major general in
both the regular army and the volunteers. At the end of the war, in 1865, George Custer was a captain in
the regular army, holding the brevet rank of major general - and at the same he was a brigadier general of
volunteers and held the brevet rank of major general of volunteers.  In 1866, after being mustered out of
volunteer service, he reverted back to his permanent rank of captain.

Brevet ranks were discontinued in 1869 but were reintroduced during the Spanish-American War and then
continued until after World War I.

At the start of the Civil War the U.S. Army consisted of a ten infantry regiments, four artillery regiments, two
dragoon regiments, two cavalary regiments, 1 regiment of mounted rifles, and various departments, such as
the Quartermaster Department. General Order No. 15 (War Department dated 4 May, 1861) detailed Lincoln's
call for 39 volunteer infantry regiments and 1 volunteer cavalry regiment. General Order No. 16 (also dated 4
May, 1861 called for several addional regiments of regulars - 8 of infantry, 1 of cavalry, and 1 of artillery. For
most of the volunteer units the officers came from within the volunteer units themeselves. These officers were
often appointed based on popularity, wealth, or social standing rather than on any military experience - or
aptitude. Throughout the war regular officers were often promoted to higher ranks, permanent or brevet, in the
volunteers as well as receiving higher brevet ranks in the regular army. In some cases this was done to allow
them to command volunteer forces, in other cases it allowed them to fill positions requiring a rank greater than
their permanent one. In almost all cases, regular officers reverted to their permanent rank after the war. Many
of the regular officers, placed in charge of volunteer units, also returned to the regular army - but not always
with their original regiment.