At German universities, students enroll for a specific program of study ("Studiengang"). During their studies, students can usually choose freely from all courses offered at the university. However, all bachelor's degree programs require a number of particular compulsory courses and all degree programs require a minimum number of credits that must be earned in the core field of the program of study. It is not uncommon to spend longer than the regular period of study ("Regelstudienzeit") at university. There are no fixed classes of students who study and graduate together. Students can change universities according to their interests and the strengths of each university. Sometimes students attend multiple different universities over the course of their studies. This mobility means that at German universities there is a freedom and individuality unknown in the US, the UK, or France. Professors also choose their subjects for research and teaching freely. This academic freedom is laid down in the German constitution.