The length from outside arm to outside arm.
From the front-most point to the back-most point. Usually from the front of the seat cushion to the back-most part of the frame.
Measurement from the floor to the top of the back cushion or frame (whichever is higher).
Measurement from the front of the seat cushion (where the welt is) to the point where the back cushion starts.
Measurement of side-to-side seating space. It's the distance between the arms, but on armless pieces it corresponds with the length.
Measurement from the floor to the top welt of the seat cushion.
Measurement from the floor to the tallest point on the arm.
Measurement from the floor to the highest point on the back of the frame.
Measurement of the skirt. It's not measured from the floor - it's just the measurement of the actual skirt because there is approximately 1" of clearance from the floor so the skirt doesn't drag on carpet. For hardwood floors, skirts can be made longer if desired.