Anibal acier au nickel pour balanciers, an alloy invented by Dr. Charles Edouard Guillaume, exhibits unusual properties, both in ther- mal expansion and change in elasticity. These properties are very different from those of two other famous alloys invented by Guillaume, Invar and Elinvar. Around 1900 Guillaume attempted to eliminate the so-called Middle Temperature Error caused by the fact that the change of rate in a timekeeper with a steel-brass bimetallic balance is approximately a linear function of temperature, while the change of rate caused by change in elasticity of a balance spring is approximately a quadratic function. Thus, it equals zero at only two tem- peratures, causing secondary error. In 1899, Guillaume noticed that steel with an addition of 44.4% nickel had a negative square coef- ficient of thermal expansion. This alloy, combined with brass in bimetallic lamina, makes its expansion close to quadratic. Balances with bimetallic rims made of anibal and brass are usually called Guillaume balances, or, as their inventor called them, integral balances. When combined with special balance springs, they exhibit remarkable temperature stability, on occasion not exceeding 1/50 second per day at 1oC.