the oriented unit normal to the surface at P. In the case of the Darboux frame along an embedded curve, the quadruple

: (P(s) = γ(s), e₁(s) = T(s), e₂(s) = t(s), e₃(s) = u(s))

defines a tetrahedron adapted to the surface into which the curve is embedded.

In terms of this trihedron, the structural equations read

Change of frame

Suppose that any other adapted trihedron
:(P, e₁,