To my knowledge Microsoft has not divulged the internal structure of indexes in any version of Access. And there are at least 4 distinct versions of the JET engine that have been released, and with 2007, 2010 and 2013 there is a new engine called ACE. The indexing technology does use "Rushmore" optimization to speed data retrieval, and there are numerous articles on both that and the engine versions that can be found with a web search.