Stellar coronae are known for their million degree plasma that is magnetically energized and emits bright X-rays. The X-ray source in black hole accretion disks is also referred to as a corona, although its observed properties, e.g., spectra, luminosity, temperature, and variability are significantly different from those of stellar coronae. In fact, the governing physical mechanisms of accretion disk coronae remain largely unknown. The colloquium will show that radio and mm observations of active galaxies (AGN) could be key to understanding the X-ray coronae of super-massive black holes. They suggest that perhaps stellar and accretion-disk coronae have more in common than meets the ... telescope.