The use of the % sign to indicate a comment is generally safer because the comment is terminated automatically by an end of line. Consequently, this form of comment is preferred.
There are two forms of usage: as a descriptor for a predicate and as comments on individual clauses and subgoals in a clause. As an illustration of comments on predicate definitions:
Now for the use of % for clauses and subgoals:
append(,L,L). % the base caseEverything on the line after ``%'' will be ignored by Prolog.
append(L1,L2,L3). % recurse on the first argument