# Axes of a markup plane

**URL:** <https://discourse.slicer.org/t/axes-of-a-markup-plane/28404>\
**Category:** Support\
**Tags:** markups\
**Created:** [March 16, 2023, 1:20am UTC](https://discourse.slicer.org/t/axes-of-a-markup-plane/28404 "2023-03-16T01:20:34Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![yshi](https://avatars.discourse-cdn.com/v4/letter/y/d2c977/32.png) [@yshi](https://discourse.slicer.org/u/yshi)\
**Post date:** [March 16, 2023, 1:20am UTC](https://discourse.slicer.org/t/axes-of-a-markup-plane/28404/1 "2023-03-16T01:20:34Z")

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Hi Slicer Community,

I created a plane with 3 markup points and would like to know how x, y, and z axes are defined. The document describes how x-axis is defined: “… the x-axis will be defined by the second (control point), and plane normal will be defined by the cross product of the vectors from the second and third points to the first point”. So I guess y-axis is defined by the third control point?

Thank you!

Slicer version 5.2.2  
Windows 11

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [March 16, 2023, 1:21am UTC](https://discourse.slicer.org/t/axes-of-a-markup-plane/28404/2 "2023-03-16T01:21:42Z")

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The y axis the cross product of the z and x axes.

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**Author:** ![yshi](https://avatars.discourse-cdn.com/v4/letter/y/d2c977/32.png) [@yshi](https://discourse.slicer.org/u/yshi)\
**Post date:** [March 16, 2023, 1:39am UTC](https://discourse.slicer.org/t/axes-of-a-markup-plane/28404/3 "2023-03-16T01:39:16Z")

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Thank you for your reply! Right, but which is defined as y (or z)? Or, when I place the third control point, am I defining the y axis? Thanks.

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [March 16, 2023, 1:58am UTC](https://discourse.slicer.org/t/axes-of-a-markup-plane/28404/4 "2023-03-16T01:58:44Z")

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For the 3-point plane placement mode:

- x axis: p1 to p2
- z axis: cross(p1 to p2, p1 to p3)
- y axis: cross(z, x)

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<div class="post-metadata">

**Author:** ![yshi](https://avatars.discourse-cdn.com/v4/letter/y/d2c977/32.png) [@yshi](https://discourse.slicer.org/u/yshi)\
**Post date:** [March 16, 2023, 4:45am UTC](https://discourse.slicer.org/t/axes-of-a-markup-plane/28404/5 "2023-03-16T04:45:21Z")

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I see, thank you so much!