The vector space of square integrable functions (with respect to Lebesgue measure) forms the Lp space with p = 2 {\displaystyle p=2} . Among the Lp spaces, the class of square integrable functions is unique in being compatible with an inner product, which allows notions like angle and orthogonality to be defined. Along with this inner product, the square integrable functions form a Hilbert space, since all of the Lp spaces are complete under their respective p-norms.