The spinning red and white pole outside a barbershop began as a medical advertisement. After the Council of Tours in 1163 barred clergy from surgery, barbers took over bloodletting, tooth pulling, and amputations. Patients gripped a wooden staff to make their veins bulge, early poles carried brass basins for leeches and blood, and bloody bandages hung to dry twisted around the pole in the wind. Painted stripes later replaced the real cloths. In Renaissance Amsterdam the colors worked as a menu, and the blue on American poles was likely a patriotic addition.

The episode follows the symbol into modern turf wars and science. The William Marvy Company in Minnesota is the last North American maker, zoning rules ban rotating signs in many places, and a 2012 Michigan proposal pitted barbers against cosmetologists over who may display one. In South Korea, paired poles can signal a brothel. The barber pole illusion, rooted in the aperture problem, inspired computer progress bars, the striped maximum speed needle that pilots fly against, NASA talkback switches that Jim Lovell called barber poled during Apollo 13, and magnetoresistive sensors in phone compasses.

• Why the maximum safe airspeed needle slides lower as a jet climbs into colder air
• The barber’s pole worm, the barber pole grasshopper, and the barber pole fish
• Sinosauropteryx, the feathered dinosaur whose banded tail was compared to a barber pole
• White Shoal Light in Michigan, Cold War border markers, and Canada’s barber pole group of corvettes
• The Shepard tone, an audio illusion called a sonic barber pole