One, unit, unity, single, solo...
An ace is number one in playing cards. French playing cards are marked '1' instead of 'A'.
A cyclops is a creature with one eye and a dromedary is a camel with only one hump.
There is only one of lots of things. There is only one planet Earth, there is only one Atlantic Ocean and there is only one you. All of these are unique.
Words beginning with uni- often mean there is one of something. For example, unicycles have one wheel and unicorns have one horn. Unisex means the two sexes appearing as one because they are indistinguishable by hair or clothing.
The letters A, B, C, D, E, M, T, U, V, W and Y all have one line of symmetry.
The international dialling code for the USA and Canada is 01.


2
2 is a prime number and is the only even prime number.
= 1 x 2 Factorial 2 or 2!
1, 1, 2, 3, 5, 8, 13... A Fibonacci number.
A deuce, a couple, a brace, a duo or a pair...
There are two blades on a pair of scissors and two sides to a piece of paper. People have two hands and so do some clocks. There are two sexes and two sides to an argument. Two-dimensional means that something has just length and width, but no depth.
Two's company, three's a crowd all depends on who you happen to be with.
Bi- means two. For example, a bicycle has two wheels and a bigamist has two husbands or two wives.


3
3 is a prime number.
= 1 + 2 A triangular number.
1, 1, 2, 3, 5, 8, 13...A Fibonacci number.
1, 3, 4, 7, 11, 18, 29...A Lucas number.
A triad, triplet, trio, tern or hat-trick...
Tri- means three. So triangles have three sides, tripods have three legs and the dinosaur triceratops had three horns. The French flag is a tricolore because it has three colours. Trigonometry is a branch of mathematics based on measuring triangles.

A triangle of triangles in the MOT test sign.
Three-dimensional means that something has length, width and depth.
There are three school terms in a year.
Oaths are traditionally repeated three times.
A three-legged race is run by two people each with a leg tied to their partner's.
The letters A F H K N Y Z are all made up of three lines.
There are three barleycorns in an inch, three feet in a yard, and three miles in a league. Barleycorns and leagues are some old imperial units of length which are no longer used today.
Once upon a time there were three little pigs ... three billy goats gruff ... Stories often begin this way and have a similar structure. Number one and number two are always similar so the listener is lulled into believing number three will be the same. But with number three there is a twist in the tale.
In Greek mythology you will find Cerberus, a three-headed dog, and Scylla, a sea monster with six heads. It is curious that mythological heads are inclined to come in multiples of three.
With just a ruler and a pair of compasses, it is possible to divide any angle exactly in half. This is called bisecting an angle. But is it possible to trisect any angle - to divide it in three - using just a ruler and compasses? Hundreds of people have spent hundreds of hours trying to discover a way to do this, without realising that it has been proved to be impossible.



4
= 2 x 2 A square.
1, 3, 4, 7, 11, 18, 29... A Lucas number.
A quartet, a foursome...
The word four has four letters. In the English language there is no other number whose number of letters is equal to its value.
The number four on a calculator is made up of four light bars.
Many things are arranged in fours. There are four suits in a deck of cards, four points of the compass, and four phases of the moon. There are four wings on a bee and four leaves on a clover, if you are lucky.
The four seasons are spring, summer, autumn and winter. This theme has provided inspiration for many artists, for the composer Vivaldi, and for countless take-away pizza establishments.
A tetrahedron is a kind of pyramid with four triangular faces. It also has four corners.

On maps adjacent countries are usually shown in different colours. What is the smallest number of colours needed? In 1852 Francis Guthrie guessed that the answer is four colours for any map, no matter what shape the countries take.
No one has ever found a map that needs more than four colours. But it has been difficult to find a satisfactory proof that only four colours are needed. In 1976 Wolfgang Haken and Kenneth Appel claimed to have proved the four-colour conjecture, but their proof is so complicated, involving hundreds of hours of calculation by a computer, it has been very difficult for other mathematicians to check.
Draw an imaginary map where the countries have very complicated shapes. Then try to colour them in with no more than four colours, and with no adjoining countries sharing the same colour. You may have to try colouring in several ways before you find a solution that requires only four colours.

A tetrahedron and a milk carton roughly in the shape of a tetrahedron.
Tetra- means four. A tetradite is someone who attaches mystical properties to the number four. A tetragram is a word with four letters (like four itself).
Quad- also means four. A quadruped is a four-footed animal like an aardvark, or almost any animal for that matter.
Plus fours are loose baggy trousers which require an extra four inches of cloth in tailoring. This ridiculous male fashion was popular with golfers in the 1920s.
In a molecule of DNA, just four bases are used to make up the genetic code that determines the distinctive form of every plant and animal. The four bases are called thymine, adenosine, guanine and cytosine, or just T, A, G and C.
Four-dimensional means that something has an extra dimension as well as length, width and depth. For the scientist, this is usually the dimension of time, where space and time are thought of as part of the same continuum.
However, in mathematics, four-dimensional means an imaginary fourth dimension in space. With two dimensions you can draw a square and with three dimensions you can make a cube. But with four dimensions it is possible to represent something called a hypercube. Some mathematicians claim to be able to visualise four-dimensional space and can conjure up a clear picture in their heads of a hypercube, which they can rotate or cut in half.



5
5 is a prime number.
1, 1, 2, 3, 5, 8, 13... A Fibonacci number.
We have five digits on each hand and foot. V, the Roman symbol for five, may originate from the image of a hand with the fingers spread.
Penta- means five. A pentathlon is an athletics contest with five events and a pentagon is a figure with five sides and five angles. A pentasyllabic word has five syllables, like the word pentasyllabic itself.
Pentagram, pentangle and pentacle are all names for a five-pointed star. This mystical symbol was supposed to keep away devils and witches. A pentacle headdress folded from fine linen was sometimes worn as a defence against demons.

A pentagon (left) and a pentagram (right).

A pentagonal bolt is fitted to many fire hydrants in the USA because it is impossible to undo with a normal spanner - most bolts are hexagonal.
A devout follower of Islam worships five times a day facing the holy city of Mecca. The Islamic creed is the `Five Pillars of the Faith'.
The Five Ks are traditionally worn by The Singhs, who are a brotherhood within the Sikh religion. These are kes, long hair; kangha, a comb; kirpan, a sword; kachh, short trousers; and kara, a steel bracelet.
There are five rings in the Olympic symbol. Basketball is played with teams of five players, and so is five-a-side football.

Five Olympic rings
Many things come in fives: the five senses, the five Chinese elements, and five vowels in the English alphabet.
In Britain, a fiver is a five pound note. In the USA, a nickel is a five cent coin.
`Take five!' means take five minute's rest.
The Five Towns, made famous in the stories of Arnold Bennett (1867-1931), are the towns of Stoke, Tunstall, Burslem, Hanley and Longton in the Staffordshire Potteries. Bennett generated some ill feeling in the townspeople of Fenton, who were left out and claimed as much right to inclusion as the other five.


6
= 2 x 3
= 1 + 2 + 3 A triangular number.
= 1 x 2 x 3 Factorial 3 or 3!
The factors of 6 (1, 2 and 3) add up to six. This makes 6 the first perfect number.
1, 2 and 3 make 6 whether you add them together or multiply them.
Sex- and hex- mean six. So there are six sides on a hexagon and six musicians in a sextet. Sextuplets are six children born together and a hexapod is something with six feet, like an insect.

A hexagonal cake box.
A cube has six faces and another name for a cube is a hexahedron. Six is the highest number on a normal die. An octahedron has six corners or vertices and a tetrahedron has six edges.
Six-legged arthropods include insects like flies, moths, ants, beetles and wasps