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This is more of a aesthetics question than maths. But if I want to use the $y$ value of a given point $P$ in an equation how to I write this? I can't really show any working since I've just googled for an answer.

Git Gud
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FraserOfSmeg
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    The important thing is, whatever notation you choose, you should include an explanatory phrase, such as “Let $P_y$ be the $y$-coordinate of point $P$” or “We will denote the $x$- and $y$-coordinates of a point $P$ as $\operatorname{proj}_x(P)$ and $\operatorname{proj}_y(P)$, respectively.” – MJD Apr 29 '14 at 14:12

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This is commonly named the projection of $P$ over the $y$-axis, denoted by $\text{proj}_y(P)$. In the link given, note in particular the bullet set theory under the applications tab.

Git Gud
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Depending on what you are using it for, you can define $P=(x,y)$ and use $y$, you can use $P_y$, you can use $\operatorname{proj}_y(P)$

Ross Millikan
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