Bugle calls

A bugle call is a short tune, originating as a military signal announcing scheduled and certain non-scheduled events on a military installation, battlefield, or ship. Historically, bugles, drums, and other loud musical instruments were used for clear communication in the noise and confusion of a battlefield. Naval bugle calls were also used to command the crew of many warships (signaling between ships being by flaghoist, semaphore, signal lamp or other means).

A defining feature of a bugle call is that it consists only of notes from a single overtone series. This is in fact a requirement if it is to be playable on a bugle or equivalently on a trumpet without moving the valves. (If a bandsman plays calls on a trumpet, for example, one particular key may be favored or even prescribed, such as: all calls to be played with the first valve down.)

Bugle calls typically indicated the change in daily routines of camp. Every duty around camp had its own bugle call, and since cavalry had horses to look after, they heard twice as many signals as regular infantry. "Boots and Saddles" was the most imperative of these signals and could be sounded without warning at any time of day or night, signaling the men to equip themselves and their mounts immediately. Bugle calls also relayed commanders' orders on the battlefield, signaling the troops to Go Forward, To the Left, To the Right, About, Rally on the Chief, Trot, Gallop, Rise up, Lay down, Commence Firing, Cease Firing, Disperse, and other specific actions.

Many of the familiar calls have had words made up to fit the tune. For example, the U.S. Reveille goes:

and the U.S. Mess Call:

and the U.S. Assembly:

Irving Berlin wrote a tune called, "Oh, How I Hate to Get Up in the Morning". In a filmed version of his musical, This Is the Army, he plays a World War I doughboy whose sergeant exhorts him with this variant of words sung to "Reveille": "Ya gotta get up, ya gotta get up, ya gotta get up this morning!" after which Berlin sang the song.

...
Wikipedia