The arms of Lannoy are blazoned: Argent, 3 hounds sable.
Geography
With a land area of only 0.18 km2 (0.069 sq mi), it is the fourth-smallest French commune (after Castelmoron-d'Albret, Plessix-Balisson, and Vaudherland) by surface area. It also has the highest density of population of any commune outside of the Île-de-France region, slightly greater than that of the city of Lyon.