A "prism" is also an open form, but the word as used here is a bit different from what we usually think of as being a prism - a three-sided bar of glass used to refract and separate light.
(A "prism; is also; an open form) (a three-sided bar of glass; used; ) (the word; used; L:here) (we; think; T:usually)
A "prism a three-sided bar of glass the word we
is also used used think
an open form
L:here
T:usually