We present a relation between the orthogonality of the constrained Legendre polynomials over the triangular domain and the BB ($B{acute{e}zier};-Bernstein$) coefficients of the polynomials using the equivalence of orthogonal complements. Using it we also show that the best constrained degree reduction of polynomials in BB form equals the best approximation of weighted Euclidean norm of coefficients of given polynomial in BB form from the coefficients of polynomials of lower degree in BB form.