Rule of the road a custom or law regulating the direction in which two vehicles (or riders or ships) should move to pass one another on meeting, or which should give way to the other, so as to avoid collision; the expression is recorded from the late 19th century.
rule of three a method of finding a number in the same ratio to a given number as exists between two other given numbers; recorded from the late 16th century, it is also called the golden rule.
rule of thumb a broadly accurate guide or principle, based on experience or practice rather than theory; the expression dates from the late 17th century.
rules are made to be broken proverbial saying, mid 20th century, often used in assertion or encouragement.
Subjects: Public Health and Epidemiology.