The part of the boundary of the lot next to a street or road is the frontage. Developers try to provide at least one side of frontage for every lot, so owners can have transport access to their lots. As the name implies, street frontage determines which side of the lot is the front, with the opposite side being the back. If the lot area is known, from the deed, then the frontage line can be calculated as depth by measuring the width (as area divided by width = depth). Sometimes minor, usually unnamed driveways called alleys, usually publicly owned, also provide access to the back of a lot. When alleys are present, garages are often located in back of a lot with access from the alley. Also when there are alleys, garbage collection may take place from the alley. Lots at the corners of a block have two sides of frontage and are called corner lots. Corner lots may have the advantage that a garage can be built with street access from the side, but have the disadvantage that there is more parkway lawn to mow and more pavement to shovel snow from. In areas with large blocks, homes are sometimes built in the center of the block. In this situation, the lot will usually include a long driveway to provide transport access. Because the shape is reminiscent of a flag (the home) on a flag pole (the driveway), these lots are called flag lots.