Thornhill has no mayor, no council and no city hall of its own. It is what planners call an unincorporated community: a place with a name, an identity and a postal address, but no government.

Its territory is divided between two municipalities that do have governments. The western portion is part of the City of Vaughan. The eastern portion is part of the City of Markham. Yonge Street is the boundary.

So when someone says they live in Thornhill, they are telling you the neighbourhood. When you need to know who runs the place, the answer is Vaughan or Markham, and you have to ask which.

Both halves of Thornhill sit within York Region, which is the upper-tier government covering nine municipalities north of Toronto, including Vaughan, Markham, Richmond Hill, Aurora and Newmarket.

That is why some services are identical on both sides of Yonge. York Region handles regional roads, water and wastewater, public health, police and York Region Transit across the whole area. The municipality handles the local services: your tax bill, waste collection, parks, libraries, zoning and permits. So a Thornhill address gets regional services from York Region and local services from either Vaughan or Markham.

1. Look at Yonge Street. West of Yonge is Vaughan. East of Yonge is Markham. For most addresses this settles it immediately.

2. Check the postal code. Thornhill addresses on the Vaughan side generally begin with L4J, and those on the Markham side generally begin with L3T. This is a useful shortcut rather than an absolute rule, so treat it as a strong hint and verify if it matters.

3. Check the tax bill or the listing. The annual property tax figure on a listing is issued by the municipality, and the municipality is named on the assessment. This is the definitive answer and it takes thirty seconds.

Thornhill began as a village on Yonge Street in the early 1800s, when Yonge was the main route north from York. The village grew on both sides of the road, but the road itself was already a boundary between two townships: Vaughan to the west and Markham to the east.

For a small village that hardly mattered. As the area urbanized through the twentieth century and municipal governments were restructured, the boundary stayed where it had always been while the community on either side of it kept growing together.

The result is a place with a genuine shared identity, including a historic main street district around Yonge and Centre Street, and no single government. Residents have never seemed especially bothered by it. It only surprises people who are new to the area.

Thornhill is large enough that "Thornhill" on its own tells you very little about a home. On the Vaughan side, areas including Thornhill Woods, Royal Orchard and Brownridge each have their own character, housing age and price range. On the Markham side, Bayview Glen, Bayview Fairway, Aileen-Willowbrook and Grandview do the same.

When you are comparing listings, compare neighbourhoods rather than the Thornhill label. Two homes both described as Thornhill can differ by decades in age, considerably in lot size and substantially in price. If you are downsizing into the area, our downsizing guide covers how to think about the move, and seniors downsizing in Richmond Hill covers the neighbouring market.