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arranged by their wavelength and frequency !
Electromagnetic (EM) waves are "arranged" according to the source of the EM radiation. The source determines orientation of the wave. Electromagnetic waves are actually composed of two waves, each of which is perpendicular to the other one. One is the electric component, and the other is the magnetic component. Depending on the source, the wave will have an "arrangement" or orientation called polarity.
In my knowledge electromagnetic spectrum is classified according to the frequency or according wavelength of EM waves-- EM waves are self propagating waves which have Electric filed and magnetic filed component which are perpendicular to each other and also perpendicular to the direction of propagation ( electromagnetic spectrum of an object is different, which depends on the chara of the object to emit or absorb EM radiation ). actually it starts from low frequency range to high frequency range... it is a continuous spectrum. But for human needs and application it used for, the frequency ranged are classified under some names....like radio waves, infra red etc..