The Valence Principle
This principle is used to allow for shorter formulations of the grammar rules.
In Sag/Wasow/Bender 2003 it is formulated as a default, i.e. as a principle that applies only if nothing else prevents it from applying.
The formulation in Sag/Wasow/Bender 2003, p.158:
Unless the rules says otherwise, the mother's value for the VAL features (SPR, COMPS, and MOD) are identical to those of the head daughter.