\[\pi\]

is a very special number, special enough to have a symbol to label it.Less special numbers have letters to label them. Unknown numbers are labelled

\[x\]

or \[y\]

, as are coordinate axes, though other symbols can be used.Integers are generally labelled

\[n\]

(the 'natural numbers' are the positive integers) and \[m\]

.Lengths can be labelled by lots of different letter:

\[a, \: b, \: c\]

are used for triangles, \[a, \: b, \: h\]

are used for trapeziums, and \[r\]

is used for the radius of a circle. \[l\]

is used to label the length of an arc - part of the perimeter - of a circle, and \[s\]

is used to lbel the slant height of a cone.