**101** (**one hundred [and] one**) is the natural number following 100 and preceding 102.

It is variously pronounced "one hundred and one" / "a hundred and one", "one hundred one" / "a hundred one", and "one oh one". As an ordinal number, 101st (one hundred [and] first), rather than 101th, is the correct form.

**101** is:

the 26th prime number.
a palindromic number in base 10, and so a palindromic prime.
a Chen prime since 103 is also prime, with which it makes a twin prime pair.
a unique prime because the period length of its reciprocal is unique among primes.
an Eisenstein prime with no imaginary part and real part of the form
3
n
−
1
.
the sum of five consecutive primes (13 + 17 + 19 + 23 + 29).
the fifth alternating factorial.
a centered decagonal number.
the only existing prime with alternating 1s and 0s in base 10 and the largest known prime of the form 10^{n} + 1.
Given 101, the Mertens function returns 0. It is the second prime having this property.

The decimal representation of 2^{101} − 1 is 2 535301 200456 458802 993406 410751. The prime decomposition of that is 7 432339 208719 x 341117 531003 194129

For a 3-digit number in base 10, this number has a relatively simple divisibility test. The candidate number is split into groups of four, starting with the rightmost four, and added up to produce a 4-digit number. If this 4-digit number is of the form 1000*a* + 100*b* + 10*a* + *b* (where *a* and *b* are integers from 0 to 9), such as 3232 or 9797, or of the form 100*b* + *b*, such as 707 and 808, then the number is divisible by 101.

On the seven-segment display of a calculator, 101 is both a strobogrammatic prime and a dihedral prime.

In mineralogy, a Miller index of 101 is a crystal face that crosses the horizontal axis (a) and 3d vertical axis (c) but does not cross the 2d vertical axis (b).
In physics and chemistry, it is the atomic number of mendelevium, an actinide.
In astronomy it is the Messier designation given to the Pinwheel Galaxy in Ursa Major.

According to Books in Print, more books are now published with a title that begins with '101' than '100'. They usually describe or discuss a list of items, such as *101 Ways to...* or *101 Questions and Answers About...* . This marketing tool is used to imply that the customer is given a little extra information beyond books that include only 100 items. Some books have taken this marketing scheme even further with titles that begin with '102', '103', or '1001'. The number is used in this context as a slang term when referring to "a 101 document" what is usually referred to as a statistical survey or overview of some topic.

Room 101 is a torture chamber in the novel *Nineteen Eighty-Four* by George Orwell.

Creative Writing 101 by Raymond Carver, "A writer's values and craft. This was what the man (John Gardner) taught and what he stood for, and this is what I've kept by me in the years since that brief but all important time."

In American university course numbering systems, the number 101 is often used for an introductory course at a beginner's level in a department's subject area. This common numbering system was designed to make transfer between colleges easier. In theory, any numbered course in one academic institution should bring a student to the same standard as a similarly numbered course at other institutions.

Based on this usage, the term "101" has been extended to mean an introductory level of learning or a collection of introductory materials to a topic.

Charter of the French Language, Bill 101.
Cantons of France, 101 Departments.
U.S. Route 101
Taipei 101, the tallest skyscraper in the world from 2004 to 2010.
101st kilometre, a condition of release from the Gulag in the Soviet Union.
The first course in a subject taught at a college or university in Australia, Canada, South Africa, or the United States. By extension, "*Topic* 101" is used generally to indicate the basics of any subject. Used this way, it is always pronounced "one-oh-one".
101 Ranch Oil Company
Roi Et Province, a province in Thailand. The name is literally 101 in Thai language.
An HTTP status code indicating that the server is switching protocols as requested by the client to do so.
For a new checking account in the US, the number of the first check.
A term used to define the number of keys on a computer keyboard
101 is the Single Non-Emergency Number (SNEN) in some parts of the UK, a telephone number used to call emergency services that are urgent but not emergencies. 101 is now available across all areas of England and Wales.
iCar 101 is a roadable aircraft design concept.
*101 Dalmatians*
The Zastava 101 is a compact car by the former Yugoslav automaker.
Highways numbered 101, the longest and most well-known of which is U.S. Route 101.
Depeche Mode - 101 (album)
Vault 101 is the starting area of Fallout 3
Lebanese Red Cross EMS Center at Spears is coded 101
In gematria, a system of assigning numerical value to a word or phrase, 101 is the numerical value of the name מִיכָאֵל (Michael in English)
101 is the identifying number of several infantry units in various militaries across the world, such as the American and Israeli paratrooper brigades
101 was the tail number of a Polish Air Force Tu-154, which crashed on 10 April 2010 whilst on its final approach to Smolensk North Airport killing all aboard including president Lech Kaczyński and his wife. See 2010 Polish Air Force Tu-154 crash.
"l'ordre des Piliers du 101" is an important student association in Belgium since 1977.
*The Wonderful 101*
Korean reality girl group survival show Produce 101