A rotation is a system by which a pipe smoker keeps his/her pipes in good working order by varying when each pipe is smoked. Most smokers own more than one pipe, their selection of pipes can be said to be their 'rotation'. It can also be said that the particular order in which the pipes are smoked is the 'rotation'. However it is used, 'rotation' speaks to the need for you to allow each pipe to rest for a period between smokes. How long your pipes need to be rested is a hotly debated topic, but it seems that the minimum time acceptable is around 48 hours, so to build a decent rotation of pipes, you will need to purchase (number of pipes you smoke in a day) x 2, to allow a two day rest between pipes.