The equivalents shown in the chart are not the only way to form mathematical analogies. In fact there are any number of ways to do this. A common requirement for analysis is that the analogy correctly models energy storage and flow across energy domains. To do this, the equivalences must be compatible. A pair of variables whose product is power (or energy) in one domain must be equivalent to a pair of variables in the other domain whose product is also power (or energy). These are called power conjugate variables. The thermal variables shown in the chart are not power conjugates and thus do not meet this criterion. See mechanical-electrical analogies for more detailed information on this. Even specifying power conjugate variables does not result in a unique analogy and there are at least three analogies of this sort in use. At least one more criterion is needed to uniquely specify the analogy, such as the requirement that impedance is equivalent in all domains as is done in the impedance analogy.