Why Does Ambiguity in a Contract Work Against Whoever Wrote It?

Here's a rule that surprises most business owners: when a contract term is genuinely unclear, a court will often read it against the side that wrote it. Not against the side that benefits, not down the middle, against the drafter. Lawyers call it contra proferentem, and it means the party who controlled the language bears the risk of any ambiguity in it. I've used this rule to win cases and watched opponents lose on it for 20 years. If you're the one handing over the contract, that's usually you, and this principle should change how you write every agreement. Here's why ambiguity cuts against the author, and how to make sure it never cuts against you.

What does "construed against the drafter" actually mean?

When two sides read a clause differently and both readings are reasonable, the court has to pick one. Contra proferentem tells the court to resolve that ambiguity against whoever drafted the language, on the theory that they had the pen, the control, and the chance to be clear, so they should bear the cost of being unclear. It usually applies when a term is truly ambiguous and one side drafted it, and it carries the most weight where there's a real imbalance in bargaining power, like a form contract handed to a customer. The practical effect is blunt: your own vague wording can be turned into a weapon against you.

Why would a court favor the side that didn't write it?

Because the drafter had every opportunity to say exactly what they meant and didn't. The other party often had to take the language as given, with little ability to change it. Putting the risk of ambiguity on the author creates the right incentive, it rewards clear drafting and refuses to let the party in control benefit from confusion they created. From the court's perspective, it's a fairness rule. From your perspective as the business that usually supplies the contract, it's a warning: the benefit of the doubt will not go to you, so you cannot afford to leave gaps and hope they break your way. They won't.

How does this play out in a real dispute?

Picture a service contract you drafted that says you'll provide "ongoing support" without defining it. The client says that means unlimited, on-call help forever. You meant occasional business-hours assistance for a set period. Both readings are plausible, which makes the term ambiguous, and because you wrote it, the court may well adopt the client's reading. Now you're contractually bound to far more than you intended, not because you agreed to it, but because you wrote it loosely. This is the everyday face of contra proferentem. It rarely shows up as a dramatic clause; it shows up as a fuzzy phrase you never thought twice about until the other side built their whole case on your version of unclear.

Does this apply to contracts between two businesses?

It can. The principle is strongest in consumer and form-contract settings with unequal bargaining power, and some sophisticated commercial contracts try to limit it with clauses stating that neither party should be treated as the drafter. But you should never rely on that as a safety net. Courts still have to interpret ambiguous language somehow, and the cleanest way to stay out of that lottery is to not be ambiguous in the first place. Whether or not the doctrine technically applies to your deal, vague language invites a fight you might lose. Clarity is the protection that works in every setting.

How do I write so ambiguity never costs me?

Write to be understood by a stranger, because a judge who has never met you is exactly who may end up reading it. Define your key terms, especially the soft ones like "reasonable," "ongoing," "promptly," or "as needed," that mean different things to different people. Use specific numbers, dates, and quantities instead of adjectives. Say what's included and what's excluded. Avoid clauses that contradict each other. And read every important provision asking, "Could someone who wanted to take advantage of me read this another way?" If the answer is yes, tighten it. The goal isn't fancy language, it's language that can only mean one thing.

Why does clear drafting beat clever drafting?

Because in a dispute, clarity is leverage and ambiguity is exposure. A clear contract tells the court what you agreed to and shuts down creative reinterpretation. A clever-but-vague contract invites the other side to argue for the reading that helps them, and if you wrote it, the doctrine may hand them that reading. The contracts that win disputes aren't the ones stuffed with the most impressive-sounding clauses. They're the ones a judge can read once and understand exactly. That's the whole game: say precisely what you mean, so no one else gets to decide what you meant.

Bottom line

Ambiguity is a risk that lands on the author, and if you supply the contract, the author is you. The protection is clarity, built in from the start by someone who knows where vagueness gets exploited. Every agreement in the Contract Library is drafted to say one thing clearly, with training that shows you how to keep it that way as you customize. Defense wins championships.

Frequently asked questions

What is contra proferentem?

It's the legal principle that ambiguity in a contract is construed against the party who drafted the language, on the theory that they controlled the wording and should bear the risk of being unclear.

Does ambiguity always get read against the drafter?

Not automatically, but often, especially with form contracts and unequal bargaining power. The doctrine applies when a term is genuinely ambiguous. The reliable way to avoid it is to not be ambiguous at all.

How do I keep my contracts from being ambiguous?

Define soft terms like "reasonable" or "ongoing," use specific numbers and dates, state what's included and excluded, avoid contradictory clauses, and reread every key provision asking whether someone could plausibly read it another way.

Can a clause saying "neither party is the drafter" protect me?

It can help in some commercial contracts, but don't rely on it. Courts still must interpret ambiguous language, so clear drafting is the protection that works regardless of whether the doctrine technically applies.

Want to legally bulletproof your business, for free? Start with the free Legal Risk Report and find your blind spots in minutes.

About the Author — Karam Nahas, The BattleTested LawyerTM. A 20-year courtroom veteran who has handled over $1 billion in deals and real litigation, Karam founded Legally BulletproofTM to give entrepreneurs the same legal defense systems big companies use — without big-law prices.

Ready to lock it down? Visit the Contract Library — every contract comes with the training and a 20-year lawyer inside your business, starting as low as $197, and it's constantly updated and customized.

Educational content, not legal advice.