An elaborate system of nomenclature has been proposed to describe, from the perspective of kinetics, the variety of enzyme mechanisms. Uni, Bi, Ter and Quad refer to the number of substrates or products (e.g. Bi Bi refers to an enzyme that is bi-reactant in both directions, and uni bi to an enzyme that is uni-reactant in the forward direction and bi-reactant in the reverse direction). Ordered describes a mechanism that has an obligatory sequence of addition of reactants (and dissociation of products), sequential a mechanism in which all reactants are added before any product is released, and random a mechanism where there is no obligatory sequence. Ping Pong describes cases in which one or more products are released before all of the substrates have added to the enzyme. Iso refers to situations in which the enzyme undergoes isomerization to another stable form during the catalytic cycle.