Or the estuary of the River Forth, that extends some 48 miles from the town of Alloa down to the North Sea on the east coast of Scotland.
To the south of the Firth lies Stirlingshire and Lothian and the lowlands of Scotland, to the north the counties of Clackmannanshire and Fifeshire and the Scottish highlands.
At its narrowest point at Queensferry are the two famous bridges;