Resonance structures are sets of two or more valid Lewis dot structures for polyatomic species possessing at least one pi bond. In resonance structures, we observe the movement of only electrons from either a pi bond—recall that double bonds and triple bonds contain pi bonds—or a lone pair. Consider NO_{2}^{−}, which is our nitrite anion. This can be depicted in one of two ways, where the oxygen on the left is double-bonded or the oxygen on the right is double-bonded. Since both configurations are possibilities, you could show both, and they are considered resonance structures. There are two resonance structures for the nitrite ion.

Notice we use double-sided arrows. These arrows are used to indicate that the resonance structures are equivalent to each other, equating one with another. The real structure is represented by the composite of these structures, often referred to as the resonance hybrid. This means that the real structure isn't exclusively resonance structure 1 or 2, but rather an average of the two.

The resonance hybrid is a composite of all significant resonance structures. To draw the resonance hybrid, we place a dotted line wherever a pi bond has been. For instance, there was a pi bond on the oxygen on the left and one on the right. Thus, we draw a dotted line at these locations to illustrate this fact. We can still place the structure in brackets and denote the charge on the outside, saying that this could represent your resonance hybrid.