Understory: the vegetation of a forest below the canopy.
Unicom System: construction system using modules as units of measure that are multiples of a standard size. Term taken from “Uniform Manufacture of Components”.
Utility Knife: also know as a boxcutter, razor blade knife, carpet knife or stationery knife. A common tool consisting of a simple and cheap holder, typically flat, approximately one inch (25 mm) wide and three to four inches (75 to 100 mm) long, and typically made of either metal or plastic. Some use standard razor blades, others specialised double-ended blades.
Valley: internal angle formed by two intersecting slopes of a roof.