Coronaviruses are large, mostly spherical, sometimes pleomorphic (changeable in shape), particles with bulbous surface projections.[42] The average diameter of the virus particles is around 125 nm (.125 μm). The diameter of the envelope is 85 nm and the spikes are 20 nm long. The envelope of the virus in electron micrographs appears as a distinct pair of electron-dense shells (shells that are relatively opaque to the electron beam used to scan the virus particle).[43][3]