## Usage as an External Aid
Usage of trade or practice is sometimes used to construe an Act.
### Underlying Idea
- The acts done under a statute often provide a key to the statute itself.
- Where the language is doubtful, the way it has been interpreted and acted upon over a long period may determine its true meaning.
- This is sometimes called contemporanea expositio — the contemporary or long-standing exposition of a statute by those who applied it.
### Practical Use
When a term in a statute is not specifically defined, customary trade usage or day-to-day administrative practice can fix its meaning.