Boykin v. State
In Boykin v. State, 818 S.W.2d 782, 785 (Tex. Crim. App.1991) the Court said that a statute is to be interpreted solely in accordance with the plain meaning of its language, unless the language is ambiguous or the plain meaning leads to absurd results.
But Boykin's strictures do not apply to the interpretation of court rules; appellate courts may consider extratextual sources even absent ambiguity or absurd results.
Nevertheless, even for court-made rules, the plain language is a good place to begin. Id. at 552.