# Expressions

ConTeXt implements extensions from eTeX including the evaluation of expressions (quoted from the [eTeX manual]):

## Expressions

eTeX introduces the notion of expressions of type number, dimen, glue, or muglue, that can be used whenever a quantity of that type is needed. Such expressions are evaluated by eTeX's scanning mechanism; they are initiated by one of the commands `\numexpr`, `\dimexpr`, `\glueexpr`, or `\muexpr` (determining the type t) and optionally terminated by one `\relax` (that will be absorbed by the scanning mechanism). An expression consists of one or more terms of the same type to be added or subtracted; a term of type t consists of a factor of that type, optionally multiplied and/or divided by numeric factors; finally a factor of type t is either a parenthesized subexpression or a quantity (number, etc.) of that type.

## Examples

It has been pointed out that the only important thing for `\dimexpr` is to write the dimen value as first value after the `\dimexpr` command and then the numeric values. Notice that the following expression is also valid:

```\placesidebyside {\externalfigure[cow][width=.4\textwidth]}
{\framed[width=.4\textwidth,height=\dimexpr.4\textwidth*200/275\relax]
{\vbox{this is a box}}}
```

 TODO: Add more illustrative examples. (See: To-Do List)