For internal reasons, Boost.Build sorts all the properties alphabetically. This means that if you write:

exe a : a.cpp : <include>b <include>a ;

then the command line with first mention the a include directory, and then b, even though they are specified in the opposite order. In most cases, the user does not care. But sometimes the order of includes, or other properties, is important. For such cases, a special syntax is provided:

exe a : a.cpp : <include>a&&b ;

The && symbols separate property values and specify that their order should be preserved. You are advised to use this feature only when the order of properties really matters and not as a convenient shortcut. Using it everywhere might negatively affect performance.