More times than I can count, when people come in looking for a very thin inexpensive frame, they reach for the fillet. When I explain to them that it isn't a frame, often times they're perplexed and annoyed. Even though fillets aren't frames, they can make a frame that much more incredible and add a touch of elegance.
So, what is a fillet? It's a very small piece of wood, about a half an inch or less, that is intended to be put on the inside of a frame or the inside of a mat. The back is flat, making it impossible for any art work to be secured to it, not to mention that it is so small and delicate it couldn't hold any art or glazing.
What's the purpose, you may ask? Why not just use a double mat to add a pop of color instead of a fillet? The answer comes down to person preference really. Some may like the look of the texture of a fillet, others may not. Some may prefer the simplicity of a single or double mat. Each piece of art requires it's own unique approach.
Fillets come in all types of designs, from modern and sleek to intricate patterns. If you love the look of a fillet, but perhaps are on a budget, some frames come with a "built in fillet," meaning that the lip of the frame (the closest part to the art) is often silver or gold with either a sleek edge or boasts a lovely design.