New to Blender, I set up mirror modifiers as part of a tute. Something now seems to have changed - when I add a mirror modifier for a plane, nothing appears in the 3D view (see screengrab). Any suggestions appreciated.
Mirroring operates around the origin point of your object (the orange dot - think of this as the centre of your object, although it could be positioned anywhere). The origin of the cube/square in your picture is right in the middle of it, so you’re actually getting a mirrored set of vertices that happen to be right on top of the vertices that were originally there. Try moving (in Edit mode) some of your vertices around. That should make what is going on obvious. To do what you want, you’ll either need to move the origin point (with the Center/Center New/Center Cursor buttons) or your vertices.
Thank you - all makes good sense, and a reminder that following the recipe in a tutorial doesn’t necessarily mean you understand much of what you’re doing!