Start with the shape of the phrase
A horn section can make one melody sound bigger without changing its notes, or it can turn that melody into a stack of moving notes. The first is often called unison playing; the second is harmony. Both can sound like a single, coordinated gesture, especially when the players match their rhythm and phrasing. To tell them apart, begin with the melody’s shape rather than trying to identify every pitch.
Listen for when the horns enter, which notes they hold and where the phrase rises or falls. If the parts seem to climb, dip and pause together, you have found their shared outline. On a second listen, ask whether you hear one pitch at each moment, colored by several instruments, or more than one pitch sounding at once. You do not need perfect pitch or a written score. This is a comparison you can make by listening to a short passage repeatedly.
What unison sounds like
In strict unison, two or more players perform the same pitches at the same time. A trumpet and saxophone may blend into a sound that is neither quite trumpet nor quite saxophone. Their different tone colors, attacks and tiny variations in timing can make the line feel thick or rough-edged, even though they are not supplying separate harmony notes. On a sustained note, try to hear whether the sound has one clear pitch center beneath those changing colors.
Jazz listeners also hear melodies doubled in octaves: one horn plays a higher version of the same line while another plays it lower. That is not strict unison, because the pitches differ, but the players follow the same note names and melodic contour. Octave doubling often sounds wide or powerful rather than chord-like. If one line seems to sit directly above the other through every turn, consider an octave before assuming the horns are harmonizing.
What changes when horns harmonize
Horns harmonize when they play different notes together. One may carry the melody while another supports it from above or below. If the parts move in a similar rhythm, the result can still feel like one line, but that line has a fuller, chord-like sound. Listen closely to a held note: can you sense a second pitch remaining alongside the first? When the phrase moves, does the distance between those pitches seem to widen or narrow?
Harmony does not always mean two lines moving in parallel. A supporting horn might hold one note while the melody moves, or enter only at the ends of phrases. Another might answer the melody in the gaps; that is a separate line, but not necessarily harmony sounded at the same moment. Pay attention to overlap. When both horns sound together, listen for the relationship between their notes. When they take turns, listen for conversation instead.
A simple way to practice
Choose a brief passage with two or more audible horns, preferably one you can replay easily. First, tap the shared rhythm without worrying about pitch. Next, hum the most prominent line. On another pass, focus on the lower-sounding horn and try to follow it through a few notes. If you can hum a different path for that part, you are likely hearing more than unison or octave doubling. If both paths seem identical apart from height, an octave may be the better description.
Try the same exercise on a sustained chord and on a fast, tightly played phrase. The held sound may make multiple pitches easier to notice; the fast phrase may draw your attention to the horns’ matched rhythm. Neither clue is foolproof. Instrument tone, room sound and the rest of the band can make separate notes hard to distinguish. It is fine to describe what you hear as “one line with a broad sound” before deciding how it is arranged.
Finally, listen for a change within the passage. An arrangement might begin with horns together on the same melody, spread them into harmony for emphasis, then leave one player to finish the phrase. Hearing that shift is more useful than labeling an entire performance as one thing. With practice, the question becomes less “How many horns are playing?” and more “What is each horn adding to this moment?”
