If the shell is considered to be in the hands of the observer, with the beaks and umbones uppermost, the right valve will always be in the right hand and the left valve in the left hand when

a. The external ligament is placed between the beaks and the observer's body; or when

b. the opening of the pallial sinus is towards the observer's body; or when

c. the centre of the single adductor muscle scar in monomyarian forms is placed on the observer's side of the midline of the shell.