Some words have an -al on the end. Some don't. Some can do either. Here are those words.
|Without -al||With -al||Mean the same?|
|scientific||scientifical||yes; the latter exists but is obsolete|
|theoretic||theoretical||where both are acceptable, yes ("set-theoretic" vs "set-theoretical"), but the latter is more widely acceptable|
|acoustic||acoustical||uses of the latter are a subset of uses of the former|
See also the superb answers to the question Why is it geometric but theoretical?