Event groups contain either events or event groups. Event groups always define contiguous sets of events. Groups can be assigned specific extents or can derive their extents from the sum of the extents of their contents. An event associates a presentable object with a particular extent within a finite coordinate space. If no object is identified the contents of the event element are taken to be the object to be presented.
Referenced objects can be in any notation.
Attributes, including data attributes associated with entity declarations, can be used to unambiguously identify the data format of the referenced object. The specifications are handled using the same mechanism employed to calibrate axes using the calibrat attribute. Scheduled extents Extents can be specified as part of an event or as a resuable HyTime component shared by multiple events. A simple dimension specification consist of two counts, the first identifying the start of the extent and the second the end of the count. If either number is negative the point being counted from is the end of the axis rather than its start.