# Definition:Triangle (Geometry)/Right-Angled/Adjacent

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## Definition

In a right-angled triangle, for a given non-right angled vertex, the adjacent side which is *not* the hypotenuse is referred to as **the adjacent**.

In the above figure:

## Also known as

Some older sources, particularly in the context of schoolbooks, refer to the **adjacent** as the **base**.

This arises from the habit of orienting the triangle (as is common in such sources) so that its opposite is a vertical line, at the side of the figure (as above).

However, confusion can arise when the triangle is positioned differently, and the angle in question is not at the bottom.

## Sources

- 1989: Ephraim J. Borowski and Jonathan M. Borwein:
*Dictionary of Mathematics*... (previous) ... (next): Entry:**adjacent**:**2.** - 2008: Ian Stewart:
*Taming the Infinite*... (previous) ... (next): Chapter $5$: Eternal Triangles: The origins of trigonometry - 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next): Entry:**adjacent side**