We field calls regularly from clients looking to have an overhead shade structure installed in their yards. Some people refer to them as pergolas. Other people call them arbors. And still others call them trellises. Are these three terms truly interchangeable? We know what you mean, but technically, each of these terms has it's own unique definition.
A pergola is what is most often sought by our clients. It is an overhead shade structure that is usually supported with some form of columns. Oftentimes, they are attached to a home, resulting in the need for less support columns than a free-standing variant. A pergola's primary intent is to provide a means of shading an area while still allowing for sunlight to enter the space. The distinguishing feature of a pergola is the use of wood members laid horizontally to create parallel lines of overhead, open, and exposed rafters. Most pergolas have the ends of the rafters cut with some ornamental shape or design to give it a pleasing and distinguished appearance. The pictures below are of a pergola we constructed recently for a client.
An arbor, while still being overhead, is typically not as large in size as a pergola. An arbor traditionally has the shape of an upside down "U" (or a lowercase "n"). It is typically covered with lattice to provide means for vegetation to grow and cover the structure. Think of an arbor as more of a "doorway". Unlike a pergola being a destination, an arbor acts as more of a transition between two distinct areas.
A trellis typically relates specifically to lattice mounted vertically for the purpose of allowing vegetative growth. A trellis may be mounted on the side of a home or it may be more of a free-standing vertical wall. A trellis can be combined with a pergola, resulting in a more enclosed space.
As designers and builders get more and more creative, these terms will continue to expand and overlap each other, so don't get frustrated trying to figure out and use the proper terms. Any experienced builder will know what you mean and provide you with your desired end result.