Surveyors must have a thorough knowledge of algebra, basic calculus, geometry, and trigonometry. They must also know the laws that deal with surveys, property, and contracts. In addition, they must be able to use delicate instruments with accuracy and precision. In the United States, surveyors and civil engineers use units of feet wherein a survey foot is broken down into 10ths and 100ths. Many deed descriptions requiring distance calls are often expressed using these units (125.25 ft). On the subject of accuracy, surveyors are often held to a standard of one one-hundredth of a foot; about 1/8th inch. Calculation and mapping tolerances are much smaller wherein achieving near perfect closures are desired. Though tolerances such as this will vary from project to project, in the field and day to day usage beyond a 100th of a foot is often impractical. In most states of the U.S., surveying is recognized as a distinct profession apart from engineering. Licensing requirements vary by state, however these requirements generally all have a component of education, experience and examinations. In the past, experience gained through an apprenticeship, together with passing a series of state-administered examinations, was required to attain licensure. Nowadays, most states insist upon basic qualification of a Degree in Surveying in addition to experience and examination requirements. Typically the process for registration follows two phases. First, upon graduation, the candidate may be eligible to sit for the Fundamentals of Land Surveying exam, to be certified upon passing and meeting all other requirements as a Surveyor In Training (SIT). Upon being certified as an SIT, the candidate then needs to gain additional experience until he or she becomes eligible for the second phase, which typically consists of the Principles and Practice of Land Surveying exam along with a state-specific examination. Typically a licensed land surveyor is required to sign and seal all plans, the format of which is dictated by their state jurisdiction, which shows their name and registration number. In many states, when setting boundary corners land surveyors are also required to place monuments bearing their registration numbers, typically in the form of capped iron rods, concrete monuments, or nails with washers.