The limits to the values of our two parameters, Pr and Pc, beyond which they make the situation unstable and provoke joint attack, are—letting h stand for the value obtained by unilateral surprise attack, –h the value obtained by being attacked while not attacking, 0 the value obtained by simultaneous attack, and 1.0 the value of joint no-attack, for each player—

DEFINE: Defines the payoff structure Pr and Pc and the threshold h beyond which unilateral surprise attack destabilizes the joint no-attack equilibrium.

Thomas C. Schelling, The Strategy of Conflict, loc. 477