 <p>It says in the <a href="http://www.boost.org/users/faq.html" rel="nofollow noreferrer">Boost Faq</a>:</p> <blockquote> <p>What do the Boost version numbers mean? The scheme is x.y.z, where x is incremented only for massive changes, such as a reorganization of many libraries, y is incremented whenever a new library is added, and z is incremented for maintenance releases. y and z are reset to 0 if the value to the left changes.</p> </blockquote>