The first digit of the course number is related to its level, or relative difficulty, of the course,[5][6] and can roughly correspond the year of study in which the course is likely to be taken. It is common for the second digit to represent the subfield in the department within which the course is offered − for example, in a Physics department, all courses numbered PHYS 47xx may be about magnetism, while all PHYS 48xx courses may be about optics. The attribution of the third and fourth digits is less standardized, but in general, sequences of related courses tend to have consecutive course numbers, with gaps between such sequences to signal different sets of courses.