Nantucket is an island. Martha’s Vineyard is an island. People are “on” islands.
Cape Cod however, is a Massachusetts peninsula, yet if one dares to say they are “in” Cape Cod and not “on” it, they will be forever scolded by locals. So why is one said to be “on” Cape Cod?
The reason for this is a that Cape Cod is a land mass and locals refer to it in that manner. They are ON the land mass that is Cape Cod. Now, when breaking it down by towns defined by geographical and political boundaries one would then say “in.” I.E. “in” Barnstable, in Chatham, in Hyannis etc.
It’s uniform for locals and those who grew up visiting the Cape, but one sure way to tell a tourist apart from a seasoned Cape go-er is whether or not they say “on Cape Cod.”
It’s similar to New York’s Long Island, which is actually a peninsula, but nevertheless people say they are “on” Long Island but “in” Easthampton.
Visitors say they are “in” Hawaii which is an actual island but “on” Cape Cod which is just a peninsula. And the hard and fast rule doesn’t truly make sense, but it must be followed by tourists and locals alike. Our word of advice-don’t overthink it, just say you’re on the Cape and enjoy your summer!