Nonstoichiometry is pervasive for metal oxides, especially when the metal is not in its highest oxidation state. For example, although wüstite (ferrous oxide) has an ideal (stoichiometric) formula , the actual stoichiometry is closer to .  The non-stoichiometry reflect the ease of oxidation of  to  effectively replacing a small portion of  with two thirds their number of .  Thus for every three "missing"  ions, the crystal contains two  ions to balance the charge. The composition of a non-stoichiometric compound usually varies in a continuous manner over a narrow range.  Thus, the formula for wüstite is written as , where x is a small number (0.05 in the previous example) representing the deviation from the "ideal" formula. Nonstoichiometry is especially important in solid, three-dimensional polymers that can tolerate mistakes.  To some extent, entropy drives all solids to be non-stoichiometric.  But for practical purposes, the term describes materials where the non-stoichiometry is measurable, usually at least 1% of the ideal composition.