The mouth of the San Francisco Bay is narrower than the bay itself.
Sounds and bays are indentations in the coastline along an ocean that take the form of naturally sheltered harbors. They are formed by glaciers, erosion and, sometimes, by the hand of man. The terms are often used interchangeably, and there are no statutory rules for naming such places in English, apart from convention. However, the habit of using the terms interchangeably can be confusing to people trying to form a mental picture of the geographic feature under discussion. There are some small differences between the two that can help distinguish a sound from a bay.
Fishing communities often base their fleets in the straits and bays as a means of providing protection for their boats.
A strait is an ocean inlet substantially larger than a bay and may be less protected. The sounds are often characterized by large open spaces of water. A sound can be deeper than a bay, and it certainly is deeper than a bay, a name for a shallow inlet of the ocean. It is also substantially wider than a fjord, an inlet formed along the coast by retreating glaciers.
On some maps, a channel or strait between two patches of land is identified as a sound; some setups require a sound to have at least two inputs. This is the case with Long Island Sound, the body of water that separates Long Island from neighboring Connecticut. It is also likely an example of the original use of the word “sound” in relation to geographical features, as the word derives from an Old English word meaning “swim”, suggesting that a person could swim in a sound.
A bay, on the other hand, is a cove enclosed on three sides by land. The mouth of a bay can be narrower than the bay itself, as in San Francisco Bay, or it can be much wider, opening out to the open ocean. The Bay of Biscay, for example, has a very wide mouth, but is still recognizably surrounded by land on three sides. The bays also tend to be shallower than the sounds, and some of them need to be dredged to allow for boat traffic.
Both bodies of water are important landforms, especially for boaters. Many fishing communities base their fleets in the straits and bays to keep their boats safe, and anchor-seeking sailors take advantage of the protected environment to take a break from the open sea. Major port cities are almost universally located along the shoreline of a strait or a bay, to provide a large docking area for the city’s trade to flourish.