From pyramids to cathedrals, no great building can exist without somebody to put it together. Masons and carpenters are similar in a number of ways. They both build things, and unlike most tradesmen, they both are mobile. You can only build so many churches and keeps in one town before you have to move on, so the builder lives in one place for a while, then moves on as his projects finish. On the other hand, a large project like a cathedral or mausoleum can employ a builder and a generation or two of descendants. Members of the lower classes usually build their own homes, so professional builders are largely employed by the upper and middle classes to build urban homes, country estates, fortifications and public buildings such as churches and guild halls.