Key-stone
It is the keystone.
ARCH-actects!
The first permanent stone- bridge was the Chain Bridge started in 1839 built to the plans of the engineer William Clark
Because it IS an arch.
The Sydney Harbour Bridge was given this unofficial nickname because its distinctive high arch shape resembles an older style coathanger.
The Daniel Carter Beard Bridge is the bridge that people call the "Big Mac" Bridge. It is nicknamed that because it is a yellow twin span steel tied arch bridge, and those yellow arches are similar to the "Golden Arches" logo of McDonald's. According to Wikipedia, in the 1980s, McDonald's considered opening a floating restaurant at the base after the nickname caught on.
or you may be thinking of your foot arch
Draw bridge
Arch
People call it the Stone Age because of the about of stone tools at the time.
The present London Bridge, in London, is a pre-stressed box girder bridge. Tower Bridge is often mistakenly referred to as London Bridge. The old London Bridge was dismantled and shipped to Lake Havasu, Arizona and is the second biggest tourist attraction in the state. The photograph displayed here is of Tower Bridge.
it is called a draw bridge