A tier is a set of annotations that share the same characteristics, e.g., one tier containing the orthographic transcription, or another tier containing the free translation.
A tier can be `independent’ and `time-alignable’, in which case its annotations are directly linked to a time interval of the media file (e.g., the `orthographic transcription’ tier). Or it can be `referring’, in which case it is linked to another tier, its so-called parent tier (e.g., the `orthographic transcription’ tier is a parent tier of the `free translation’ tier). The referring tier shares its time alignment with its parent tier. The annotations of some referring tiers can be assigned to the time axis, but only to an interval that is contained within the interval of their parent annotation.
It is possible to build nested hierarchies, e.g., the `orthographic transcription’ tier is the parent tier of a `word’ tier, and the `word’ tier is the parent tier of a `morpheme break’ tier.