Prefixes, usually of Latin and Greek origin, are placed at the beginning of a word to modify its meaning. Those relating to numbers are called numerical prefixes. Below is a table of some of the prefixes for one to ten:

Numerical prefixes can help you make an educated guess at unfamiliar words. Take the word dilemma as an example. Starting with the prefix di-, it describes a situation in which one must choose between two options. You should then be able to figure out the meaning of trilemma, which refers to a quandary in which a tough choice has to be made among three alternatives rather than two.

Numerical prefixes can also be found in adjectives about frequency of occurrence, such as biennial and triennial, which mean recurring every two and three years respectively. Biannual and triannual, on the other hand, mean occurring twice and three times a year respectively. Different numerical prefixes can be used to form words with similar meanings, such as quadrennial (recurring every four years), quinquennial (happening every five years) and decennial (occurring every ten years).

The prefix bi- often trips people up because it can either mean “occurring twice” or “once every two”. Therefore, biweekly can be “twice a week” or “every two weeks”. Bimonthly can be “twice a month” or “every two months”. To avoid the confusion, it is better to use more precise expressions or leave some clues in your context to indicate which meaning is being referred to.

In writing, there are specific terms for age groups by decade. A person aged between 60 and 69 is sometimes called a sexagenarian. Likewise, the word septuagenarian refers to someone in their seventies, while octogenarian refers to a person aged 80 to 89.

A glance at the prefixes for seven to ten may leave you puzzled about the names of months: Shouldn’t September, October, November and December be the seventh to tenth months in a calendar year? In fact, they used to be. Originally there were only ten months on the Roman calendar, with March being the first month of the year and December the tenth. Later January was added to the beginning and February to the end of the calendar to name the winter months and create the 12-month year. In 452 BC, February was moved to its current position between January and March. September to December, the seventh to tenth months, were thus pushed down and became the ninth to twelfth months. Their prefixes, however, remained unchanged.