In security printing, void pantograph refers to a method of making copy-evident and tamper-resistant patterns in the background of a document. Normally these are invisible to the eye but become obvious when the document is photocopied. Typically, they spell out "void", "copy", "invalid" or some other indicator message. Void pantographs work by exploiting the limitations and features of copy equipment. There are many patents in use for this technology, each works slightly different.