Unprotected Opening : A vertical or horizontal opening through a floor, wall, or other partition in a building that allows the passage of smoke, heat, and flame
Ralph W. Burklin & Robert G. Purington, Fire Terms: A Guide to Their Meaning and Use, National Protection Association, Boston, MA 1980 USA. (Guide: Meaning and Use, Nat Prot Association Fr Srvc - Amer) {457}