Eight corners rule is a principle applicable to Insurance law. According to this rule, a liability insurer's duty to defend its insured is assessed by reviewing the claims asserted in the plaintiff's complaint, without reference to matters outside the four corners of the complaint plus the four corners of the policy.
The following are examples of caselaw discussing the rule:
This rule requires the finder of fact to compare only the allegations in the underlying suit, i.e.,the suit against the insured with the provisions of the insurance policy to determine if the allegations fit within the policy coverage. Courts must apply the eight corners rule liberally and resolve any doubts in favor of the insured. If any allegation in the complaint is even potentially covered by the policy then the insurer has a duty to defend its insured. However, courts must not read facts into the pleadings, look outside the pleadings, or imagine factual scenarios which might trigger coverage.[Std. Waste Sys. v. Mid-Continent Cas. Co., 612 F.3d 394 (5th Cir. Tex. 2010)]
When following the eight corners rule, a court must recognize that the duty to defend is broader than the duty to indemnify because it arises whenever a complaint alleges facts and circumstances, some of which, if proved, would fall within the risk covered by the policy. If it is doubtful whether a case alleged is covered by the policy, the refusal of the insurer to defend is at its own risk. And, if it be shown subsequently upon development of the facts that the claim is covered by the policy, the insurer necessarily is liable for breach of its covenant to defend. [Fed. Hill Homeowners Ass'n v. Cmty. Ass'n Underwriters of Am., Inc., 2010 U.S. App. LEXIS 12689 (4th Cir. Va. June 21, 2010)]