What's the difference between int2 and int16?

I found the uses of int2, int16 and other similiar types misleading
in PostgreSQL's source code. Sometime it is difficult to figure out
which should be prefered.

Maybe int2, int4, and int8 refer to database types, while int16, int32
and int64 refer to C data types. If this is the convention, maintenance
may be easier.