The membranous pa of the interventricular septum is situated in between:

Membranous pa of ventricular system is a small poion of the ventricular and atrioventricular septum located at the base of the hea adjacent to the anteroseptal tricuspid commissure , the right aoic posterior valve commissure and the anterior mitral valve leaflet. Because of the apical displacement of the tricuspid valve compared to mitral valve, a poion of the membranous septum separates the left ventricle from the right atrium. The remaining three regions of the ventricular septum are inlet, outlet and trabecula. They are all muscular and and they radiate out from the membranous septum. Ref: Echocardiography in Pediatric and Adult Congenital Hea Disease edited by Benjamin W. Eidem, page 196.