Layer of protection analysis (LOPA) is a simplified chemical process risk assessment tool that is used to determine if sufficient independent protection layers (IPLs) are in place to meet an organization's risk tolerance criteria for a particular hazardous scenario. LOPA can determine how much additional risk reduction is needed for the scenario and can help in specifying the additional protection layers. The Center for Chemical Process Safety books, Layer of Protection Analysis-Simplified Process Risk Assessment (Center for Chemical Process Safety, Layer of Protection Analysis: Simplified Process Risk Assessment, American Institute of Chemical Engineers, New York, NY, 2001) and Guidelines for Safe and Reliable Instrumented Protective Systems (Center for Chemical Process Safety, Guidelines for Safe and Reliable Instrumented Protective Systems, American Institute of Chemical Engineers, New York, NY, 2007), identify required attributes of independent protective layers. In more than 15 years experience working with LOPA, I have frequently observed confusion about these attributes among LOPA analysts, protection layer implementers, and operations personnel (Study and Champion, Process Saf Prog 28 (2009), 300-307). Therefore, this article discusses the characteristics of IPLs, gives examples to avoid confusion and misapplication of the characteristics, gives an easily remembered phrase to screen candidate IPLs, and provides sample tools to confirm IPL compliance with the characteristics. The article discusses the importance of a safety requirements specification for safety-instrumented functions and proposes analogous documentation for other IPLs. The article gives guidance to determine when protection layers or scenarios require more detailed analysis such as event tree analysis or fault tree analysis. (C) 2011 American Institute of Chemical Engineers Process Saf Prog 30: 126-131, 2011