The eight-corners rule is a rule applied by courts to determine whether an insurance company (insurer) has a duty to defend a claim made against its insured policyholder (insured). The eight-corners rule provides that the duty to defend is determined by comparing the “four corners” of the plaintiff’s pleading (lawsuit) with the “four corners” of the liability insurance policy.

In applying the eight-corners rule, courts generally do not consider facts or evidence from outside the four corners of each of these documents and take the plaintiff’s factual allegations in the pleading as true for purposes of determining whether the insurer has a duty to defend.

But some courts have held that outside or extrinsic evidence may be considered if it demonstrates collusion or fraud between the plaintiff and the insured for the purpose of invoking an insurer’s duty to defend.

Courts generally apply the eight-corners rule liberally and resolve any doubts in favor of the insured by finding the insurer has a duty to defend the insured against the claim(s).

In California, the eight-corners rule is used to determine if an insurer has a duty to defend its insured in a lawsuit. This rule involves comparing the lawsuit's allegations (the 'four corners' of the plaintiff's pleading) with the terms of the insurance policy (the 'four corners' of the policy). California courts typically do not consider facts outside of these documents when applying the eight-corners rule. However, they may consider extrinsic evidence if it indicates fraud or collusion between the plaintiff and the insured, aimed at triggering the insurer's duty to defend. The rule is applied in a manner that favors the insured, meaning that if there is any ambiguity or doubt, courts are likely to err on the side of finding that the insurer has a duty to defend. It's important to note that while the eight-corners rule is a general guideline, specific case law and statutory provisions in California may also influence how this rule is applied in practice.