In combat, flags serve a practical purpose as a means of identifying unit locations and as a rallying point for soldiers in the confusion of battle. Flags are also used to identify specific individuals, and mark important locations such as unit headquarters and field hospitals. Flags provide a sense of identity for a unit and build pride and morale
Guiding and referencing troops in battle was a key function of the guidon, as explained by cwartillery.com. Artillery units during the Civil War depended upon the guidon for reference as to where the line of battle was being set and as a rallying point in the heat of attack. The guidon bearer would position himself at the right, left or center of the designated line of battle approximately 35 yards behind the guns so that everyone else knew where to form up and where the line was at. So that he was visible to all troops, the guidon bearer rode a horse. While marching, the guidon was a "guide," so to speak, for the officers directing their troops, knowing where the line was being moved and how to turn their men when orders were given
Cavalry standards and guidons were flown from nine-foot long staffs capped with a brass spear point at the top and a brass butt cap on the bottom. Color Bearers would attach a small leather cup or “boot” to the stirrup leathers on the off side of their saddle (see post on saddles below) to facilitate carrying the flags while mounted.
During the course of the War, corps, divisions, and brigades adopted non-regulation flags to mark the location of their headquarters. Several systems to standardize these headquarters flags were attempted. In 1862 Major General George B. McClellan devised a system of red, white, and blue flags and flags divided into bars of red, white, and blue to designate various higher headquarters. Numbers added to the flags distinguished the regiments within a brigade. McClellan’s complex, confusing system was replaced in 1863 by a simpler system that identified commands by the shape of the flag. Corps headquarters were designated by a swallow-tailed flag, divisions by a rectangular flag, and brigades by a triangular pennant. Within a corps, divisions were differentiated by use of the distinctive corps badges developed earlier in 1863 by Major General Joseph Hooker. A red badge on a white field distinguished the 1st division, a white badge on a blue field the 2nd division, and a blue badge on a white field the 3rd. Within divisions, brigades were designated by the borders of their triangular flags. A plain pennant with no border denoted the 1st Brigade; a stripe along the “hoist” of the pennant denoted the 2nd brigade, and a border on all three sides of the pennant the 3rd brigade. This model gradually became the standard for armies in the east and was adopted with some variation by the western armies when the 11th and 12th Corps were transferred to Tennessee to reinforce General Ulysses Grant late in 1863.
The significance of the guidon is that it represents the unit and its commanding officer. When the commander is in, his or her guidon is displayed for everyone to see. When he leaves for the day, the guidon is taken down. It is an honor, although sometimes a dubious one, to be the guidon carrier for a unit, known as a "guidon bearer" or "guide". He or she stands in front of the unit alongside of the commander (or the commander's representative), and is the rallying point for troops to fall into formation when the order is given. In drill and ceremonies, the guidon and commander are always in front of the formation.
The guidon is a great source of pride for the unit, and several military traditions have developed around it, stemming back from ancient times. Any sort of disgrace toward the guidon is considered a dishonor of the unit as a whole, and punishment is typical. For example, should the guidon bearer drop the guidon, he must fall with it and perform punishment, often in the form of push-ups. Other units may attempt to steal the guidon to demoralize or antagonize the unit. Veteran soldiers know not to give up the guidon to anyone outside their unit, but new recruits may be tempted into relinquishing it by a superior, especially during a unit run.
This style of guidon was used in the Indian Wars up until 1885.
This style of Guidon was used in the Civil War and then again after 1885 to present.
Traditionally, the carrying of a Guidon, Color, or Standard remained the exclusive privilege of those who fought face to face with the enemy, namely the Cavalry and the Infantry. Originally they were battle flags, carried by the headquarters staff to show the position of the King, Lord, General or unit commander. For that reason they were always cut with a swallow tail design so that they would flutter better in the breeze, which actually made it easier for Troopers to recognize. It is a tradition that has been followed by many countries with mounted units.
In addition to the regimental standard, individual cavalry companies carried swallow-tailed flags called “guidons.” At the beginning of the Civil War cavalry guidons featured two horizontal bars, red over white. In 1862 the regulations changed and cavalry guidons featured red and white stripes with a blue canton in the same design as the National colors. Although the regulations did not authorize cavalry regiments to carry the National colors, many did, carrying either a scaled-down version similar in size to their standards, or a swallow-tailed guidon in the pattern of the National colors, but without company or regimental designations painted on.
The U.S. Cavalry used guidons in the Civil War as well as the Plains Indian Wars later on. The Cavalry were the last of the three branches of service of the U.S. Army to get to carry the Stars & Stripes in battle - Artillery was first in 1836, then Infantry in 1842 and the Cavalry at the start of the Civil War.
Infantry also carried a smaller guidon (called a Flank Guidon) one on a pole that would fit into the barrel of a rifle and was carried by the last soldier in a column while his rifle was at shoulder arms on the march. This flank guidon was for the 4th Infantry Regiment:
The chronology of the Amy Guidon: