Width (When fully opened) x Hinge Length
Butterfly hinges attach to the outside of box so the full hinge is visible when the box is closed. Butterfly fittings are basically a butt hinge though they are designed to be fancy shapes rather than simple.
They are also more of a decorative piece than general purpose one and aren't as strong as butt hinges so they are used on smaller things that are made to be more aesthetically pleasing like jewellery boxes.
Note: These hinges close to 90° and open up to 225°.