Possible Clues:

• A clarifying comparison
• A hypersensitivity, say, regarding correspondence
• SAT staple
• Former SAT question type
• A : B :: C : D, e.g
• Correspondence, some enclosing a journal
• A diary kept in whatever relationship helping to explain
• Verbal comparison
• One often has four colons
• "A is to B as C is to D," e.g