When drawing the shoulder area of the torso, watch for the “arc of hollows.”
These pits or indentations follow a curving line from the pit of the neck to beneath the clavicle. The line curves along the leading edge of the deltoid and ends in the armpit.
The arc of hollows is marked in yellow on this study of a reference sculpture.
The hollows are marked with X’s here in Michelangelo’s David.
This idea came from Drawing the Head & Figure, by Jack Hamm, 1963, page 70.