The phrase opinion letter denotes a broad category encompassing many types of lawyers’ letters with various purposes and formats. Examples abound: usury opinions in loan transactions, title opinions in real-estate transactions, closing opinions in securities offerings, coverage opinions in insurance matters. Such a letter is almost always a written opinion of law regarding a decision to be made or a plan of action to be taken. An opinion letter prepares the client to make an informed decision.
Clients seeking opinions want to know their rights, obligations, and potential risks. They want to know about any possible legal consequences they may face in a given situation. An opinion letter needs to answer the question posed, and it needs to be well written in language the client can understand. If it’s obscure and indefinite or sounds like legaldegook, clients will rightly judge the writer harshly and resent paying for it. But if it shows both care and a warm interest in the client’s affairs, and it’s in plain English, a thoughtful opinion letter can help cement the lawyer–client relationship.
Further reading: The Redbook: A Manual on Legal Style 451–64 (4th ed. 2018).