Construction liens are also sometimes known as mechanic's liens. Historically, the term "mechanic" once referred to any person who performed skilled labor, not limited to current usage that assumes a machine to be the subject of that work. Thus, at the time the phrase "mechanic's lien" was invented, the understanding was that such a person might be a carpenter, plumber, or the like. Because of the change in the meaning of the word "mechanic," some states have changed the statutes to have a "mechanics lien" for people who work on cars and the like, and a separate "construction lien" statute to deal with construction-related payment disputes. The term "lien" comes from the French root (via William the Conqueror), with a meaning similar to link; it is related to "liaison."
What type of contribution counts as a valid basis for a construction lien is also variable. The core purpose is protecting the benefit that a worker provides, such as the time and effort a carpenter puts into nailing the boards together on the job site, and thus is included in the scope of most liens. However, other types of contributions are less direct - the contribution of an architect, or the supply company that delivers materials, or a company that rents the backhoe to the contractor, or the company that rents the port-a-pots to the contractor, or the truck that brings food to the workers at lunchtime. There is no simple dividing line that is useful in every state, or even in every case. Often, determining whether a party has a legitimate lien right depends on examining other cases that have either upheld or rejected lien claims in the same state.
Mechanics liens are a reaction to the imbalance of power between a worker at a construction site, and an owner of that land. The worker makes the time and effort investment on the assumption that the owner will pay, but until the owner does pay, the owner is in a significantly superior power position. The improvements have already been made, and it will not significantly benefit the worker to demolish the work. Thus, unscrupulous owners could simply lock the tradesman out of the property, retain the benefit, and refuse to pay. Additionally, as a society we benefit by having improvements to buildings, and knocking them down as a resolution to disputes is economically inefficient. Because of the difficulties inherent in contract suits, most clearly time and cost, states decided to provide a simpler procedure for putting pressure on an owner to pay a claim, short of executing a judgment.
While the construction lien is overall a benefit to the worker, there are protections in the process for the owner. Generally, the worker must follow a strictly constrained process, and failure to follow that process will invalidate the lien. Some parts of that process are intended to prevent disputes from occurring, such as a structure of mandatory notices and disclosures that provide the owner an opportunity to ensure that the project's finances are being properly managed, in addition to being able to monitor the physical progress of the work.
Property of the government is ordinarily not subject to the claims of private parties, and a purported construction lien against government land is generally void. To protect subcontractors and suppliers of US federal government construction projects where the contract price exceeds $100,000.00, the Miller Act (40 U.S. Code 3131) requires general contractors to give a surety bond which guarantees payment for work done in accordance with the terms of the contract. Many states and municipal governments also require contractors on public works projects to be bonded.