Ideally, a timescale in which the sequence of geologic-time units and their corresponding time-rock divisions (chronostratigraphic units) are defined by standard and internationally agreed reference points within boundary stratotypes. In practice, boundary stratotypes have been agreed only for parts of the scale and the chronostratigraphic units within it continue to be defined by biostratigraphic means. See standard stratigraphic scale; stratigraphic scale.