Except as prescribed by 28-11-105, a guaranty must be in writing and signed by the guarantor, but the writing need not express a consideration.
Mont. Code Ann. § 28-11-104
Guaranty generally to be in writing -- form
En
Official source: Montana Code Annotated (Montana Legislature). Reproduced from public-domain Montana statutes; confirm against the official source for the current text. Not legal advice.