Types of Flags
In the Union Army, each regiment carried two flags, a "national" flag and a "regimental" flag.
The national flag was the flag of the United States. It sometimes bore special adornment, such as the name of the regiment or the names of the battles the regiment had participated in painted on the stripes. Unlike modern flags, the stars were very often painted on in gold. Pennsylvania regiments inserted their state seal into the star-field. The flag was also decorated with hanging tassles. The infantry used tassles of twisted blue and white, whereas the artillery used tassles of red and yellow.
The regimental flag varied from state to state. The regimental flags of Regular Army infantry regiments were blue and bore the seal of the United States. This was also the predominant design among volunteer infantry regiments (although some regiments used a variant of their state flag, or a unique design representing that particular regiment). Artillery regimental flags were yellow, and bore the artillery's crossed cannon-barrel devise.
Although the regulations specified national and regimental flags only for regiments, it was not uncommon for individual batteries to carry their own set of national and regimental colors, particularly in "independent" batteries which were not raised as part of a parent regiments.
The most familiar symbol of an artillery battery was the guidon. The guidon was a small, swallow-tailed United States flag. The stars were usually painted on in gold and arranged in a circular pattern with the letter of the battery in the center.
As with so many other things from that time period, many other designs and variations existed.
Regulations for Army Flags
COLORS OF ARTILLERY REGIMENTS.
1465. Each regiment of Artillery shall have two silken colors. The first, or the national color, of stars and stripes, as described for the garrison flag. The number and name of the regiment to be embroidered with gold on the centre stripe. The second, or regimental color, to be yellow, of the same dimensions as the first, bearing in the centre two cannon crossing, with the letters U. S. above, and the number of the regiment below; fringe, yellow. Each color to be six feet six inches fly, and six feet deep on the pike. The pike, including the spear and ferrule, to be nine feet ten inches in length. Cords and tassels, red and yellow silk intermixed
1467. The camp colors are of bunting, eighteen inches square; white for infantry, and red for artillery, with the number of the regiment on them. The pole eight feet long.
STANDARDS AND GUIDONS OF MOUNTED REGIMENTS.
1468. Each regiment will have a silken standard, and each company a silken guidon. The standard to bear the arms of the United States, embroidered in silk, on a blue ground, with the number and name of the regiment, in a scroll underneath the eagle. The flag of the standard to be two feet five inches wide, and two feet three inches on the lance, and to be edged with yellow silk fringe.
1469. The flag of the guidon is swallow-tailed, three feet five inches from the lance to the end of the swallow-tail; fifteen inches to the fork of the swallow-tail, and two feet three inches on the lance. To be half red and half white, dividing at the fork, the red above. On the red, the letters U. S. in white; and on the white, the letter of the company in red. The lance of the standards and guidons to be nine feet long, including spear and ferrule.
Taken from the Revised United States Army Regulations of 1861