a security against poor quality products (defects) or works left incomplete. Clients withhold retention against main contractors and main contractors withhold payment against sub-contractors. Retentions typically take the form of a percentage on the contract value. The rate can vary wildly but is typically around 5%. The general state of the economy can affect the rates set: in a buoyant economy with plentiful work sub-contractors are able to pick which work they accept and therefore have potential to negotiate more favourable rates. The chain of retention starts with the client who withholds money on the main contractor. The main contractor withholds money on sub-contractors who may also then withhold on sub-sub contractors. The retention money is typically released in two portions (known as moieties); the first being payable at completion of a project and the second at the end of the defects liability period. This period is the time during which the client is able to identify works that are defective to the contractor who must then remedy them; it is often twelve months. The use of retentions is not common to all sectors of the industry; for example lift installers have developed their own guarantee system instead. A mobilization payment is an advance payment to a contractor at the start of a project to assist in the beginning of operations. Impact The use of retentions is intended to encourage efficiency and productivity. The contractor has a financial incentive to achieve completion as early as possible (to release the first moiety payment) and to minimise defects in the works (to achieve the second payment). Retentions held against sub-contractors are also a key source of cash for main contractors, who may use them to finance new projects. However sub-contractors often complain about the system. They sometimes lack a firm date on which retention monies will be paid and a 2017 British government report noted that more than half of contractors had experienced late or non-payment of retention monies. Delays are reportedly longer for sub-contractors and sub-sub contractors than for the main contractor. This restricts cash flow available for the company as a going concern and for capital investment. The chasing up of payments is also resource intensive, as such smaller businesses are hit more severely than larger ones. Some smaller companies simply write off the retention money, increasing their prices to compensate. The practice has also been described as increasing tensions