Business Letters (Chapter 9): Formal correspondence with clients, vendors, regulators. Essential parts: Date, recipient address, salutation, body (clear, concise message), closing (professional sign-off), signature. Types: Inquiry letters, quotation requests, proposals, complaint letters, follow-up letters. Format: Block format (standard business), indented (traditional). For accountants, business letters critical for client communication and compliance. Exam tip: Follow standard format; maintain professional tone; ensure completeness.