The representation of the structure of a molecule by two or more conventional formulae. For example, the formula of methanal can be represented by a covalent structure H2C=O, in which there is a double bond in the carbonyl group. It is known that in such compounds the oxygen has some negative charge and the carbon some positive charge. The true bonding in the molecule is somewhere between H2C=O and the ionic compound H2C+O−. It is said to be a resonance hybrid of the two, indicated by H2C=O↔H2C+O− The two possible structures are called canonical forms, and they need not contribute equally to the actual form. Note that the double-headed arrow does not imply that the two forms are in equilibrium.