Here at the Curiosity Desk, we love nothing more than digging for answers to the everyday mysteries that catch your eye. And recently, Sarah Rambacher — a resident of Littleton, Mass., and self-described "lover of books and libraries" — reached out with an email that caught our eye.

I would love to know why the town line signs in Massachusetts are shaped like a book. Is it our history in education and libraries? Is it even a book?

Surely you have seen the signs Sarah is talking about. When I read her email, I immediately knew what she meant, though I must admit it never really occurred to me that these ubiquitous signs were actually in the shape of a book.

These unique signs are almost as old as many of the roadways where they are seen. They were originally designed and developed during the highway boom of the early 1930s by the Massachusetts Department of Public Works — the predecessor to the MassDOT Highway Division.

And it is indeed a book. Highway officials refer to them as the "bookleaf" town line sign. As for whether this particular shape was chosen — at least in part — as a nod to the Commonwealth's long literary tradition? That seems to be lost to history. But one thing we know for sure is that they were made to look distinct.

"The town line signs were designed with the unique 'bookleaf' shape in order to help drivers distinguish the signs from other directional signs," said Maxwell Huber with the Massachusetts Department of Transportation in an email. At the time, Huber said, many other directional signs "also had black legends on white backgrounds and used a 'cut corner' shape."

Still curious? Check out Curiosity Desk on YouTube

The earliest versions were made of plywood and mounted on concrete posts. Today, it's sheet aluminum on tubular steel posts. Despite the upgrade in materials, Huber said, "the basic design of these signs has changed little since their original inception."

You'll note that the "bookleaf" town line signs are used only on secondary U.S. and state numbered highways, or on non-numbered roads or streets. Sign standardization has come a long way since the 1930s. Today, all signs on interstates and freeways must comply with the Federal Highway Administration's Manual on Uniform Traffic Devices. So on the biggest, busiest roads, town signs — and plenty of others — are that familiar white lettering on a green background.

Another feature of these signs is that each includes an established (EST) date or an incorporated (INC) date on them. Huber said the incorporation date is standard, but an established date will be swapped in if so requested by a municipality for historical reasons (plenty of towns in the Bay State were settled for years before officially incorporating). As for where those dates come from? They are provided by the Secretary of State’s office.