Opened by the Queen in 1964, it replaced ferries which had plied the river since at least the 11th century.
Taking six years to build, it was the largest suspension bridge outside the United States, more than 1.5 miles long including approach viaducts.
The design was so novel a special cable-spinning training school was established in South Queensferry. The main cables contain 30,000 miles of steel wire – enough to reach around the world.
Increasing traffic volumes and maximum lorry weights doubling to 44 tonnes have prompted mounting repairs and maintenance, including strengthening the towers in 1990 and 1998.
The cost of the bridge was repaid in tolls by 1993, but they continued to fund bridge work until being abolished after the SNP was elected, in 2008.
The bridge became a category A listed structure in 2001 and carries 24.5 million vehicles a year – twice its expected limit.