How do you mirror across a user defined axis?

Hi All,

I’m trying to mirror a curve across an axis that is not one of the 3 predefined axes. In Autocad I would select mirror, then draw the mirroring line & the object would be copied across, that’s the function I’m looking for. It’s a curve which needs to be mirrored at a set distance away & I have the centre line marked, so I’m not keep on copying & flipping it & then moving it around, although if that’s the only way then I guess that’s what I’ll have to do! I thought this would be a basic function but can’t seem to find it anywhere, perhaps I’m searching for the wrong thing.

Here’s the project I’m working on, the selected line is what I’d like to mirror & the small line to the left of it is where I hope the mirroring axis will be.

Thanks very much.

With Rotate tool:

  • activate Rotate tool
  • click, hold and drag along the center line of mirroring
  • rotate (180°) + ctrl (for copy)


Here you can get away with dragging the ‘Rotate’ tool along the edge because it is a “2D” selection that you want to mirror.

Take advantage of the real mirror functionality in SketchUp 2023.
Temporarily set an axis along the edge, then apply the ‘Flip’ tool and hit the [Ctrl] once to optain a mirrored copy.

Original situation

Mirror (axis) set along the edge

Mirrored copy using green mirror slided into position

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Click in sequence on the scenes tabs of this SU file for ideas.

Mirror along a line.skp (343.9 KB)

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It looks like in your example you’re still flipping it along one of the predefined axes though? You say to use the flip tool & that’s exactly what I’ve been doing & it doesn’t seem to work. I’m not sure how to ‘temporarily set an axis’ as you instruct - what command would you put in to do that?

Amazing, it’s taken me a long time to figure out exactly what you did but it worked! Thank you so much.

I use the ‘Axes’ tool Screenshot 2023-06-14 11.30.48 with that I first click on one endpoint of that short edge and double click on the other endpoint. This way the red axis will be along the short edge (and green perpendicular) and blue still pointing up.
With these axes the next step is to apply the ‘Flip’ tool. Its green flip plane will be according to the new temporarily set drawing axes: i.e. green flip plane perpendicular to the green drawing axis.

The rest is just about how to apply the ‘Flip’ tool on a selection:

  • make select
  • select ‘Flip’
  • either hit [Ctrl] to obtain a mirrored copy of the selection (or just mirror the selection)
  • tab (or here drag into position towards the short edge) the appropriate color of the mirror (here green plane).

So basically two things:

  • set up axes (like in any other software where you may need to click on a face or edges)
  • apply the ‘Flip’ tool

p.s. setting up different oriented drawing axes can also be beneficial with other tools like the ‘Dimensions’ tool.