Partial inner product spaces and semi-inner product spaces

A comparison is made between the two objects mentioned in the title. Connections between them are threefold: (i) both are particular instances of dual pairs of locally convex spaces; (ii) many partial inner product spaces consist of chains or lattices of semi-inner product spaces; (iii) the basic structure behind both of them is that of Galois connections. A number of common open problems are described.