Why Mixing two signals f1 and f2 produces products at combinations like 2f1-f2 and 2f2-f1 (third order, an odd order). If f1 and f2 are close together, those odd-order products land just above and below the original pair, inside the same band or passband. Even-order products like f1+f2 or f2-f1 land near twice the operating frequency or down near the difference, far away from the originals and easy to filter out. That is why third-order intermodulation is the spec that matters for receiver and amplifier linearity.
Watch out Second harmonics are a form of even-order product, appearing at twice the input frequency rather than near it; intercept point is a measurement of how strong the distortion products are, not a frequency location.
Odd order = odd man close by. 2f1-f2 sits right next door; even-order products run far away.