I am new to using Blender, so please excuse me if I use the wrong terminology to describe my problem.
I am half way through modelling an ancient brass door knocker and seemingly from nowhere the wire frame outline of a cube has appeared, the edges being dotted. Non of the items I used to generate my model were cubes, and before starting work I deleted the cube in the default Blender file.
The mystery cube has positioned itself immediately over where I added my initial plane when starting the project and has the same orientation. It is showing as selected and will only deselect in object mode; it automatically reselects in all other modes.
It does not, however, appear in a rendered image.
The cube refuses to be deleted and there are no vertices showing that I can deal with individually.
Can someone please tell me what kind of object this cube is, where it may have come from, and most importantly how to remove it.
Many thanks.