The Sprint is the fundamental unit of work when using Scrum. Any product development effort using Scrum is, therefore, divided into Sprints. Sprints are fixed in length so that the team has a predictable amount of time available to them to do work, which in turn assists in both short and long-term planning. By making every Sprint the same length, the Scrum Team learns its own capacity for work. If the Sprint length changes, the rhythm of Scrum is broken and a team will have to re-learn its capacity which usually takes at least a few Sprints. If Sprints are rarely the same length, then the Scrum Team will struggle to do any reliable planning.