This article explains about expression in Small Basic.  In Mathematics, there are three types of expression - identify, equation and definition.  But in Small Basic, there are two types of expression - substitution and condition.

# Substitution

Substitution is equality which heft-hand side is a variable or a property of an object and right-hand side becomes a value. The value is assigned into the variable or the property.  Following three lines are all substitutions.

`gw ````= ````598`
`GraphicsWindow``.````Width ````= ``gw`
`a ````= ````2 ``* ` `Math``.````Pi ````* ``r`

In Small Basic, right-hand side of substitution equality is not allowed to use boolean operation (And or Or), equality or inequality.

# Condition

Condition is described in If statement or While statement.  Condition is an equality, an inequality, a boolean variable, or an expression with boolean operation(s) which becomes a boolean value ("True" or "False").  In following lines, condition is (0<= x) And (x < gw) And (0 <= y) And (y <= gh).

```If ````(``0``<````= ````x``) ````And ````(``x ````< ````gw``) ` `And ``(````0 ````<``= ``y````) ````And ``(````y ````<``= ``gh````) ````Then`
`  ````color ````= ``GraphicsWindow``.``GetPixel``(``x````, ````y``)`
`EndIf`