Re: "Position Parameters" in lispref/frames.texi

[Richard Stallman]

    FWIW, the first "+" and "-" discriminate left vs right (top vs bottom)
    screen edges/frame borders.  For example, "left +" specifies the left
    frame border "left -" the right one.  (I don't understand why we can't
    use right and bottom here.)

It would be natural to use `right' and `bottom' if these were just
arguments to a function.  But they are frame parameters.  If we want
the horizontal position to have two possible frame parameters, `left'
and `right', we would have to arrange to return the proper one, etc.
It would be rather complex.