commercial vehicles and buses must be equipped with the device per law Straßenverkehrs-Zulassungs-Ordnung § 57a. Tachographs are mandatory for vehicles allowed to carry a total weight of over 3.5 tonnes and vehicles built to carry at least 9 passengers, if the vehicle is used for commercial purposes. They are used to review the driving and rest time of drivers during reviews by traffic standards organizations or accident investigation. A driver must carry the tachograph records with him for all days of the current week and the last day of the previous week that he drove. Companies must keep the records for 1 year. In Germany, § 16 of the work time regulations lengthens this time to 2 years if the records will be used as proof of work time. In the EU EEC regulation 3821/85 from 20 December 1985 made tachographs mandatory throughout the EEC as of 29 September 1986. (Regulation 1463/70 amended by regulation 2828/77 made tachographs mandatory by 1 July 1979, reference to these regulations can be found in regulation 3821/85). A "European arrangement in regard to the work of driving personnel engaged in international traffic" (AETR, from French "Accord Européen sur les Transports Routiers") became effective on 31 July 1985. Regulation 561/2006/EC of the European Union adopted on 11 April 2007 specified the driving and rest times of professional drivers. These time periods can be checked by the employers, police and other authorities with the help of the tachograph. Analogue tachographs Most tachographs produced prior to 1 May 2006 were of the analogue type. Later analogue tachograph head models are of a modular design, enabling the head to fit into a standard DIN slot in the vehicle dashboard. This would enable a relatively easy upgrade to the forthcoming digital models, which were manufactured to the same physical dimensions. The analogue tachograph head uses styli to trace lines on a wax coated paper disc that rotates throughout the day, where one rotation encompasses a 24-hour period. If the disc is left in the head over 24 hours, a second trace will be written onto the first, and so on until the disc is removed. It is an infringement of EU Regulation 561/2006 to use a disc for a period longer than it is designed for. Multiple overlapping traces may still be deciphered in the speed and distance fields, but it is far more difficult for the activity field