Diaulos (Greek: Δίαυλος, English translation: "double pipe") was a double-stadion race (ca. 400 m.) introduced in the 14th Olympiad of the ancient Olympic Games (724 BC). Scholars debate whether or not the runners had individual "turning" posts for the return leg of the race, or whether all the runners approached a common post, turned, and then raced back to the starting line.
^Miller, p. 32. "The first addition to the Olympic program was the diaulos, or double-stadion, race in 724. This race, the functional equivalent of the modern 400-meters, is difficult to identify on the vase paintings, because the runners' gait is similar to that of stadion runners. By comparing runners whose knees are slightly lower than the others' (contrast the knees in figures 3 and 11 with those in figure 30), we may be able to distinguish between the two events. The only depiction of the diaulos of which we are certain, however, is on a fragment of a Panathenaic amphora labeled, "I am a diaulos runner"."
^Golden, pp. 51-52. "Diaulos, "double pipe" (see AULOS), one of the regular events at Greek competitive festivals. The diaulos was a footrace two lengths of the STADIUM (400 Greek feet) - up in lanes marked out by lime or gypsum, counter-clockwise around a post (KAMPTËR), and back in a parallel line. According to tradition, it joined the Olympic programme second, in 724; the first winner was HYPENUS of Pisa. Only men ran diaulos at Olympia, Isthmia and Nemea. At the Pythian games, however, it was on the original programme for both paides and men. Diaulos runners needed to be stronger than sprinters but lighter than HOPLITE racers (who ran the same distance, in armour); however (according to Galen), even a champion would be slower at this distance than a gazelle. Unchallenged by gazelles, some diaulos runners enjoyed success over many years: an Argive won at Olympia four times running (208-196), a feat matched and surpassed by LEONIDAS of Rhodes (164-152, adding victories in STADION and race in armour as well). Double victories in stadion and diaulos were not uncommon. The term is also used for a horse race of two lengths of the Greek HIPPODROME."