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.