There were a number of types of headgear worn during the Civil War. The above illustration does not depict the full range or both regulation and nonregulation wear. In fact, head may have varied more from regulation than any other uniform item. The chapeau was little used except for formal wear. Most officers did not own one. It did see some use:
At 12 o'clock noon General Keyes, accompanied by Couch, Peck, and Graham with their respective staff, called upon President Lincoln. General Keyes was dressed in his ordinary uniform with shoulder straps, trouser tucked into his boot tops and with cover on his forage cap. The other generals worn full dress uniforms, chapeaux, and presented a marked contrast to General Keyes.
--Elisha Hunt Rhodes, 2nd Rhode Island Infantry The hat was worn as dress. It is given the name Jeff Davis hat because he was the prewar Secretary of War at the time of its adoption. A more comfortable version of the hat was the so-called Burnside Pattern or Variation. This was named after General Ambrose Burnside. Interestingly enough photographs of Burnside do not suggest that he commonly wore this style hat. The differences between the common officer's "slouch hat" and the Burnside may become blurred when looking at a large numbers of hats. The forage cap or kepi came in the standard model or a lower cut braided Chasseur pattern. The light artillery cap was worn by some units as dress, but not used in the field. Aside from these common military styles many soldiers chose to wear civilian style hats in the field.