Why does joining two objects change the position of another object?

Question

Here I have a left armor piece, the body, and a right armor piece. The left armor piece and the right armor piece used to be a mirrored object but I have separated them. I'm currently trying to join the left armor piece to the body, however when I join them the right side changes. At first, I used CTRL+J to join, but that created a new armor piece on the right. Then I tried to use the boolean tool, but that results in the right armor piece being moved. How can I join the Left armor piece without adding or moving other objects?

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Answer 1

The reason for this is the "nature" of join.

If you join 2 or more objects, Blender has to decide where the pivot point is for the newly created object. Blender takes as new pivot point the last selected object. So "normally" by joining two or more objects you will change the pivot point (exception: if all pivot points have the same global coordinates). But the mirror modifier calculates the mirroring on the base of the pivot point of an object (if you didn't choose a mirror object). You can prove that by moving the pivot point of a mirrored object e.g. by object -> set origin -> to 3d cursor.

If you would have used an object/empty in your mirror modifier as mirror object this wouldn't happen because Blender uses then the object instead the pivot point.

Here a small animation how changing the pivot point changes the result of a mirrored object: