In the modern common parlance, many people use the word “bugle” to refer to any brass instrument of a moderate size that is just a coiled tube and a bell with no valves or keys. This is particularly true in the United States. This is inaccurate, however, and would include several different brass instruments. Particularly, the trumpet did not have any valves until the mid to late 19th century. The existence or lack of valves is not the defining feature.
The pedantic answer is that a bugle is a lip reed instrument with a generally conical bore throughout its length. A trumpet is a brass instrument with a generally cylindrical bore through most of its length until the bell flare begins. This shape of the bore, which affects the timbre of the sound, is the defining feature.
This is an important distinction to understand when looking at, and discussing particular instruments. However, we must also recognize that common usage should be considered and pedantry is not always useful. I will tend to use bugler in this generic sense, regardless of instrument, but will endeavor to accurately use bugle and trumpet when referring to the physical instruments themselves.
Traditionally, the bugle was used by dismounted troops (e.g. foot infantry) and the trumpet by mounted troops (e.g. cavalry). Military manuals from the time when buglers were employed in combat are very clear on the distinction between bugles and trumpets. Cavalry manuals refer to trumpeters, not buglers. Manuals written to cover all services refer to both trumpeters and buglers and would commonly state “buglers and trumpeters”’ or “bugles and trumpets.” It was around the start of the 20th century that US manuals started to use only one term to mean both, with some upfront, one-time statement that the use of “bugler” referred to both buglers and trumpeters (or vice versa).
To add to this confusion, we can also consider the word “horn.” In the general sense, this is used to refer to any brass instrument of any type. Especially in the upper registers. However, horns are also a particular type of instrument. For example, the French Horn.
Trumpets were different before the adoption of valves and the creation of our modern concert trumpet. These early trumpets were roughly twice the length of our modern trumpet. They had a tubing length similar to a modern trombone, but with a bore diameter closer to our current trumpet. This had the effect of lowering the fundamental pitch an octave but allowing trumpeters to easily play in the upper harmonics. Essentially, the first line E becomes an open note playable with no valves. This is similar to the modern French Horns, but the trumpet had a different (cylindrical) bore profile.
The bugle (or bugle-horn), however, was not a longer instrument. Its tubing was roughly half that of its trumpet counterpart (when in the same key).
This confusion is particularly a problem in the United States. Europe seems to have maintained the nomenclature of “bugle” and “cavalry trumpet” or “herald trumpet” when discussing these two valveless instruments. The United States, however, seems to have shifted to calling anything without valves a bugle. Yes. Our beloved and iconic “bugle” in G, adopted by the Boy Scouts of America, chosen by American Legion posts when the competition Drum Corps movement began, seen in so many movies over the last century, and used as a model for the Duty Bugler logo, is, in fact, a trumpet. The official nomenclature from the US Army Quartermaster specification is “M1892 Field Trumpet.” The US Army also had a bugle counterpart to the M1892- the “M1894 Bugle” in Bb. Two years younger than the M1894 Field Trumpet, this bugle is the last signal instrument specified by the US Army for field use