In the custom framing industry, a fillet is a small piece of moulding which fits inside a larger frame or, typically, underneath or in between matting, used for decorative purposes. The picture framing term is probably related to, though not necessarily derived from, the engineering term, which it is frequently pronounced similarly to; however, unlike the use of fillets in mechanical engineering, the use of "fillets" in picture frames is wholly decorative.  (Wikipedia)

Pronunciation

Fillet can be pronounced in two ways.

One way is to pronounce it as if it were "fill-it", as the similar term from mechanical engineering is pronounced. The other is similar to the French-derived culinary term. Either is acceptable in English, though most frame shops prefer one or the other pronunciation.

Construction

Fillets are typically made of soft or hard wood, and feature a flat "lip" which can fit underneath a mat; the non-lip portion is what is displayed. Except for their shape and size (which is understandably small), fillets are constructed similarly to picture frames, usually from wood or polystyrene. Metal fillets are very rare. Fillets are available in a number of styles and finishes, including gold and silver leafed finishes