The central feature of the Roman army of the mid-Republic, or the Polybian army, was the manipular organization of its battle-line. Instead of a single, large mass (the phalanx) as in the Early Roman army, the Romans now drew up in three lines consisting of small units (maniples) of 120 men, arrayed in chessboard fashion, giving much greater tactical strength and flexibility. This structure was probably introduced in c. 300 BC during the Samnite Wars. Also probably dating from this period was the regular accompaniment of each legion by a non-citizen formation of roughly equal size, the "ala", recruited from Rome's Italian allies, or "socii". The latter were approximately 150 autonomous states which were bound by a treaty of perpetual military alliance with Rome. Their sole obligation was to supply to the Roman army, on demand, a number of fully equipped troops up to a specified maximum each year.