A mechanics' lien is a type of lien that secures the priority of payment for work or materials expended in erecting or improving upon a building or land. This type of lien protects a contractor's prepayment investments in another person's land, i.e. the contractor's materials and labor, by allowing the contractor to obtain a lien interest in the property that is the subject of the improvements. Mechanics' liens are a unique type of lien, separate from other types of liens - such as judgment liens - in that a mechanics' lien is a claim against the property itself ('in rem'), rather than against the person himself ('in personam'). Mechanics' liens are purely statutory, did not exist at common law, and therefore are strictly construed, requiring strict compliance with the statutory procedures, otherwise the lien may be lost.