T-unit ('terminable unit') is essentially a main clause with a non-elliptical subject, including any dependent clauses
Thus When I got home, the new TV I ordered yesterday was on the doorstep, is one T-unit,
as is John got up and left the room. However, John got up and Mary left the room is two T-units
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But in order to constitute a proper variable a good categorisation/classification system should:
be exhaustive,
have mutually exclusive categories,
not mix categories of different types in one set,
have unambiguously defined categories,