“Quoins” are the exposed stone pieces that you sometimes see stacking up only on the corner of a building. Their use today is odd and usually fake and are trying to allude to traditional masonry construction and presumably the sense of durability, solidity and timelessness that implies.
Quoins provide a kind of emphasis, a visual boldness, to the corners of a building and tend to make the building feel more solid, more object-like. However, like so many elements of architecture that appear to be merely stylistic touches, they have an origin in a construction technology.
Quarrying stone has always been a difficult and expensive proposition. Making a stone building out of the stone that are scattered around the field and forest is a much easier proposition but results in a random rubble type wall. When that random rubble wall has to turn a corner, the stone of differing sizes and shapes create a visual and technological problem. Because of its ragged line it collects water, because of the use of small, varied stone, they easily pop off the corner when exposed to the elements from two sides. Quoins of cut stone were used to contain the edges of stone walls and help solve these exposed corner problems. Being cut stone they stack nicely and cleanly on each other and their consistent size and shape they solidly and securely hold the corner true and vertical.
However, this technological use of quoins has long been forgotten and they are merely stylistic touches now applied without much subtlety to buildings. You can see fake stone quoins, face brick quoins, wood quoins trying to look like stone quoins, and best yet, EIFS (fake stucco) quoins, in buildings all over the country.