HBO is therefore the equivalent of college education in the United States. After HBO (typically 4–6 years), pupils can enroll in a (professional) master's program (1–2 years) or enter the job market. In some situations, students with an MBO or a VWO diploma receive exemptions for certain courses, so the student can do HBO in three years. The HBO is taught in vocational universities ("hogescholen"), of which there are over 40 in the Netherlands, each of which offers a broad variety of programs, with the exception of some that specialize in arts or agriculture. Note that the "hogescholen" are not allowed to name themselves "university" in Dutch. This also stretches to English and therefore HBO institutions are known as universities of applied sciences. The WO ("wetenschappelijk onderwijs"; literally "scientific education") is only taught at research universities. It is oriented towards higher learning in the arts or sciences. After the bachelor's programme (typically 3 years), students can enroll in a master's programme (typically 1, 2 or 3 years) or enter the job market. After gaining a master, a student can apply for a 3 or 4 year PhD candidate position at a university (NB a master's degree is the mandatory entry level for the Dutch PhD program). There are three technical universities, an Open University, six general universities and four universities with unique specializations in the Netherlands, although the specialized universities have increasingly added more general studies to their curriculum. A national system of education was introduced in the Netherlands around the year 1800. The "Maatschappij tot Nut van 't Algemeen" ("Society for the Common Good") took advantage of the revolutionary tide in the Batavian Republic to propose a number of educational reforms. The Education law of 1806 encouraged the establishment of primary schools in all municipalities and instated provincial supervision. It also introduced a mandatory curriculum comprising Dutch language, reading, writing, and arithmetics. History, geography, and modern languages such as French, German and English were optional subjects. All newly established schools needed consent from the authorities or would be disbanded as freedom of education was not proclaimed until the Constitutional Reform of 1848.. In addition to primary education, gymnasia (or, Latin schools) and universities constituted higher education. What could be considered secondary education or vocational training was unregulated. This situation changed in the second half of the nineteenth century in the wake of social and economic modernisation. In 1857, a Lower Education law replaced the 1806 law supplementing the mandatory curriculum with geometry, geography, history, natural sciences, and singing.