While one and a hundred have a huge difference in quantity, the words “one” and “hundred” often go hand in hand in idioms or fixed expressions. A hundred to one describes things with a low probability of happening: There is a hundred to one chance of rain today. As an idiom, a hundred and one means very many. In these expressions, the word “hundred” is interchangeable with “thousand” or “million”.

It is perplexing that 101 (pronounced “one-oh-one”) and a hundred and one are not synonymous. In the United States, college course codes are usually expressed in three-digit numbers, with the first denoting the year of study and the last the sequence of classes. Introductory courses are therefore designated a code number 101. Nowadays the number is no longer confined to a college setting but widely used in an extended sense. When placed after a noun, 101 refers to the basic facts or knowledge of something: To avoid colour distortion, washing light and dark clothes separately is laundry 101.

Geometry 101: a semicircle measures 180 degrees, and a full circle is 360 degrees. The distinction between 180 and 360 in language usage is, however, easy to overlook. In a figurative sense, doing a 180 (or doing a one-eighty) means a complete reversal in views, attitudes or positions: The committee members were in favour of the motion, but they went on to do a 180 and voted against it. But doing a 360 (or doing a three-sixty) simply means making a 360-degree spin: My car slid, did a 360, and eventually came to a stop.

As an adjective, 360-degree carries the figurative meaning of “comprehensive”: The deadline for this 360-degree report falls on the coming weekend.

Despite the seemingly straightforward expressions with numbers, making an educated guess is easier said than done. While a hundred per cent is another expression of “absolutely”, it also means healthy and fully fit: She did not feel a hundred per cent. In mathematics, 100% and 110% are two different percentages, but, in English usage, the expressions give a hundred per cent and give 110 per cent both convey the meaning of making the fullest efforts.

Given this line of thought, it is tempting to presume that keep it 100 is a proverb encouraging one to make the best efforts or stay in good shape. The real meaning, however, is staying true to oneself or behaving honestly and sincerely. According to the Oxford English Dictionary (OED), this colloquial expression is probably the shortened version of “keep it 100 per cent real”. Despite its increasing popularity in recent years, it was not entered into the OED until 2024.