Several extensions of the basic modal language are characterized in
terms of interpolation. Our main results are of the following form:
Language ℒ' is the least expressive extension of ℒ with
interpolation. For instance, let ℳ(D) be the extension of
the basic modal language with a difference operator [7].
First-order logic is the least expressive extension of ℳ(D)
with interpolation. These characterizations are subsequently used
to derive new results about hybrid logic, relation algebra and the
guarded fragment.