A float frame is used with canvas artwork. It differs from a standard frame in that the art is placed into the float from the front and shows the entire canvas edging. A standard frame has a lip on it which holds the art into the frame, and covers the edges of the canvas.
Float frames are more common now than in the past and come in a variety of sizes, shapes and depths to accommodate a variety of canvas heights.
Some float frames fit tightly around the canvas they frame and others have some space around them, that can be chosen by the framer to allow for breathing space around the canvas. I prefer to decide the amount of space around the canvas, depending upon the image and the depth of the canvas I am working with, or what effect I am trying to achieve.
Sometimes a float frame is used alone to create a minimal edge to enclose the artwork and other times it is used with another frame edge to be used as part of the overall design.
Sometimes you want to be able to see all of the canvas, but a float is not a solid enough edge for the frame, so we layer the framing to enhance the art or bring out the personality of the artwork. Typically we use a float when the art has nicely finished edges or wraps around the canvas, although I have taped edges of canvass when its not a particularly expensive piece, but the look of a floater is requested.