Not all orbits were possible, according to Bohr, but only those that met certain conditions. Although his original conditions were more complicated, in essence he proposed that the only stable electronic orbits were those that corresponded to standing, or stationary, electron waves around the orbit, with an integral number of complete wavelengths. (See illustration on the right)

The orbit with the smallest size and lowest energy would have one complete electron wave around its circumference; the next allowable orbit would have two complete waves, and then three, four, and so on.

An intermediate orbit that corresponded to 2.25 or 5.33 waves per circuit would lead to destructive interference between waves from successive turns around the orbit, as shown on the right.