What type of promise must be documented in writing if it involves paying another's debt?

A promise to pay another's debt must be documented in writing if it is classified as a collateral promise. This type of promise arises when one party agrees to pay the debt or obligation of another, typically as a secondary obligation, which is separate from the primary debt owed by the original debtor. Under the Statute of Frauds, such promises must be in writing to be enforceable. This requirement safeguards against disputes and misunderstandings regarding the agreement's existence and terms.

In contrast, express promises are explicitly stated, either orally or in writing, but they do not specifically pertain to a third-party's debt. Implied promises are inferred from the actions or circumstances surrounding the parties and typically do not require written documentation. Conditional promises hinge on specific conditions being met, yet they too do not typically require written form unless they fulfill the criteria set out in the Statute of Frauds. Thus, the written documentation is essential for collateral promises to ensure clarity and protect the parties involved.