What I mean is, if we are considering M^{*} \otimes M \otimes M^{*} which means M \times M^{*} \times M \rightarrow \mathbb{R}, so the argument is of the from (x,f,y) where f are linear forms, so since f are elements of M* and x,y is in M, then why don't we write M \otimes M^{*} \otimes M instead?