Boston was originally a near island, connected to the mainland by a very narrow isthmus. The narrow strand was called Boston Neck. All foot traffic to and from Boston entered via the neck; shortly after its founding, the city built a defensive gate there.
The neck was so narrow that British solders dug a trench across it during the occupation of Boston at the outset of the Revolutionary War, turning Boston into a true island, for a while!
Here's a colonial map of Boston; I've indicated the neck with an arrow.
No trace of the original Neck remains today, but I was curious. I went looking for maps that would superimpose the colonial city on a current ,map, but could find none. Some old maps in the Boston Public Library offered hints, but also added confusion:
The original main street in and out of Boston changed names along its way: It began as Marlborough Street, changed to Newbury Street after a few blocks, and then changed again to Orange Street. After the Revolution, the street was given a unified name --- Washington Street.
Marlborough Street, Newbury Street, and Orange Street vanished, for a while.
Wikipedia says the Neck was about where this red circle is, at the intersection of what's now East Berkeley and Washington Streets:
For clarity, I outlined the original shore:
This begins to show how much of Boston is reclaimed land: The original city was something like 4 square miles; it's now about 40 square miles (10sq.km/100sq.km).
Marlborough Street and Newbury Street eventually came back to life as new streets in the reclaimed section of Back Bay, wholly unconnected to their original locations. There's an Orange Street miles away, in another town; but the only remnant of Orange Street that remains in Boston proper is the name of the "Orange Line" subway!
And now we both know a little more useless trivia about Boston. :)