Full Idea
A predicate letter followed by one name expresses a property ('Gm'), and a predicate-letter followed by two names expresses a relation ('Pmn'). We could write 'Pmno' for a complex relation like betweenness.
Gist of Idea
'Gm' says m has property G, and 'Pmn' says m has relation P to n
Source
E.J. Lemmon (Beginning Logic [1965], 3.1)
Book Reference
Lemmon,E.J.: 'Beginning Logic' [Nelson 1979], p.98