Vincenc Strouhal, 1878, walking past a telegraph line, hears the wires sing. He gets curious in a particular way. He sets up flow past wires of various diameters and measures the frequency of the tone. What falls out of the data is a single number — later named for him — about 0.2. The relation is f = St · U / D: pitch equals strouhal number times wind speed divided by wire diameter. Nothing about the wire's tension. Nothing about the air's viscosity. Nothing about its density. Just speed and width.
What's singing isn't the wire. The wire is being shaken sideways by something happening downstream of it. When fluid passes a cylinder fast enough, it can't stay attached around the back; the boundary layer separates, rolls into a vortex, and is shed off one side. That single shed vortex breaks the symmetry of the wake. The asymmetric pressure field forces the next vortex to form on the other side. Which, when it sheds, breaks symmetry the other way. The wake becomes its own clock.
This pattern — alternating vortices peeling off downstream in a regular train — is the kármán vortex street. It needs nothing to drive it but throughput. There is no external oscillator. The first asymmetry, whichever side it happens on, is enough; the geometry takes it from there. What looks like a periodic forcing of the wire is really the wake exhaling first to one side and then to the other, and the wire is along for the ride.
That Strouhal's number is roughly constant across five orders of magnitude of Reynolds number is the part I keep coming back to. The same dimensionless relation governs a singing fence wire, a swaying chimney, an industrial cooling tower, a marine riser on an oil platform, the tail of a fish holding station in a current. Different fluids. Different scales. Different bodies. Same number. The wake doesn't care about most of what you'd think it would care about. It cares about width and speed.
The engineering implication is that anything cylindrical in a flow has a frequency, and if that frequency happens to coincide with one of the body's natural modes, the wake will pump energy in resonantly until something fails. Tacoma Narrows wasn't this — that was aeroelastic flutter, a different beast — but bridge cables can do it, and chimneys do it often enough that they're frequently wrapped in helical strakes. The strake isn't trying to stop the vortices; it's trying to keep them from shedding coherently along the whole length, because coherence is what couples to the structure.
So: the named thing isn't the wire. The wire is the visible body that catches the eye. The actual mechanism is two layers in: a wake that has learned to break its own symmetry, and a number that holds across nearly every scale at which the trick works.