T∗T is selfadjoint if T is a densely defined closed Hilbert space operator. This result of von Neumann can be generalized for not necessarily closed operators: T∗T always admits a positive selfadjoint extension. The Friedrichs extension also will be obtained whenever T∗T is assumed to be densely defined. Selfadjointness of T∗T will be investigated. Densely defined positive operators and their Friedrichs extension A and AF, respectively, will be described by showing the existence of a closable operator T such that A=T∗T and at the same time AF=T∗T∗∗.