# Position of points relative to a plane

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Harold on 16 May 2013
Commented: Diego Hens on 21 Aug 2020
I'm looking for a way to decide whether a point is on, to the left, or to the right of a plane. I have it to work for a selected point. For example, given a number of (x,y,z) coordinates, I'm able to determine what points are on, or across the a line drawn from a point say x = 3. The code I developed works perfect. However, I want to expand on this so that any plane can be used not just the yz plane (x = 3). I want to do a batch check on all the points in a variable called vertices. There will always be three point coordinates in this variable.
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### Accepted Answer

Yao Li on 16 May 2013
1. Obtain the equation of the plane
2. If the point (x,y,z) satisfy the equation, the point is on the plane. I don't think you can use the word 'left' and 'right' to describe the position before you have specified the reference axis.
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Diego Hens on 21 Aug 2020
Did you find a solution to this? I have the same problem. I want to erase all points under a plane from an stl

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### More Answers (1)

Jan on 16 May 2013
If the plane is defined by one point P and the normal vector N, is is easy to decide on which side (defined by the orientation of N) the point Q is: check the sign of:
v = dot(Q - P, N);
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