The OpenMRS community can only support up to three a limited number of released versions at a time (the current release and then two versions back). The previous versions are kept in folders called 1.5.x or 1.6.x and only small bug fixes are applied to those branches. This makes those branches relatively stable and very similar to all releases in that minor version (e.g. 1.5.1 will be very similar to 1.5.9), since supporting a release takes peoples' time and effort (e.g., backporting bug fixes). As a community, we have agreed to do our best to support up to three versions. This means we strive to maintain the current release along with the two prior minor versions.