Like most agile methods, Space takes an iterative approach to project work. Iterations are a great way to facilitate reflection, improvements and strike a good balance between having “work peace” to get stuff done, reflecting upon that work and receiving feedback from other stakeholders (e.g. customers). A yardstone in Space is similar to a sprint in Scrum except that the content is determined by the sum of the estimates for the yardstone content and the duration of the yardstone is the actual completion of that content. A Yardstone is thus not a hard timebox like a sprint but instead a “feature box”. I.e. what is fixed for the duration is not the time but rather the set of functionality. That being said, the time dimension is also important and typically there is a yardstone every 2-4 weeks. Yardstones that are longer than 4 weeks does not promote iterative practices and are not recommended. The purpose of a yardstone is to enable and support continuous improvement by providing a natural rhythm in project work (like a breath cycle) where review and reflection is given appropriate room at suitable intervals. This means that in the beginning of a project it is quite common for the duration of the yardstone to vary a lot from the original estimate. This, as opposed to fixed length sprints in Scrum, greatly helps the team to improve estimation skills as it becomes very obvious after each yardstone how much over or under the original estimate the team actually was, providing a great opportunity for improvement of this vital skill. A rule of thumb is to select functionality that is estimated to take about 3 weeks, but for the initial 5 iterations target 2 weeks instead.
A milestone is a specific set of discernable functionality that is achieved by completing several yardstones.