Called locus communis in Latin and topos koinos in Greek, commonplaces
are, according to Aristotle, the seats of arguments or pigeonholes of
the mind where one could find material for an oration (Lechner 1-2). In
this sense, the commonplace resembles what we might today call a
heading. Used for the sake of argument, these headings allowed an
orator to divide a topic into its many parts, and would typically
include definition, cause, effect, opposites, likenesses as well as
others. In addition to these classifications, locus communis has
refered to collections of sayings (in effect, formulas) on various
topics--such as loyalty, decadence, friendship, or wha- tever--that
could be worked into ones own speech-making or writing (Ong, 111).
These two meanings of commonplace Ong refers to as analytic and
cumulative. If the commonplace book is an example of the cumulative, it
is informed by the analytic.
-- Paul Dyck "Reading and Writing the Commonplace:
Literary Culture Then and Now" (Re)Soundings (Winter 1997)
http://www.millersv.edu/~resound/*vol1iss1/topframe.html