"It’s one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man." -Richard Feynman The fine structure constant (Greek letter α), which governs the strength of interactions in quantum electrodynamics (and whose precise value is one of the fundamental reasons you are able to exist at all), is a derived constant with a value of approximately 1/137. In any system of units. Being a ratio of other fundamental constants (namely: the elementary charge, Planck's constant, the speed of light, and either the permittivity or permeability of free space, depending on how you want to write it), the units on the top and bottom cancel out to resolve in a pure number. It is as yet unclear why it has the value it does, but if it were significantly different in either direction complex matter (and, by extension, you and everything you've ever cared about) would not exist. It is, in that sense, perhaps the most important number there is. 1/137