The Seine is crossed by 37 bridges along its 13 km course in Paris. Four serve the Ile Saint-Louis and eight the Ile de la Cité. The Invalides bridge is the lowest and the Alexandre-III bridge is the widest, the most majestic and the most decorated. The oldest structure is the Pont Neuf, completed in 1607. It bears this name because it was the first stone bridge and the only one that did not support houses. Until 1968, the Pont Neuf was the longest bridge (232 metres). The Aval bridge, used by the Boulevard Périphérique, became the longest bridge in Paris at 312.50 metres. Nine bridges are classified as historical monuments.