As I was going through some old West Elm catalogs, I came across some very savoy information about curtain lengths. So here it is, direct from one of the greatest stores in existence, West Elm.
A. Sill Length. "A casual look, with curtains grazing the windowsill. Practical with windows that open frequently, and especially cute when you hang panels cafe-style, about halfway below the moulding, to let in more daylight."
B. Apron Length. "Curtains fall 4" below the sill. Ideal for kitchens and baths."
C. Floor Length. "Curtains float about 1/4" above the floor, for a crisp, polished feel. This is the ideal way to hand panels with detailed borders on them."
D. Extra-Long. "Curtains that graze the floor by a few inches or pool in a romantic puddle of fabris are perfect for rich fabrics like velvet."
*Images from westelm.com