Derivational morphology often involves the addition of a derivational suffix or other affix. It does not affix items to such surfaces permanently. In linguistics, the term affix refers to either a prefix or a suffix. The epenthetic vowel "e" may be inserted between the affix and the verb. Candidates or their polling agents may also affix their own seals to the ballot boxes. Some authorities affix to the back sides of signs. Alternative techniques exist that affix the implant without cement. A bottle cage is device used to affix a water bottle to a bicycle. Dados are often used to affix shelves to cabinetry carcasses. In simpler terms, a null morpheme is an "invisible" affix. Certain affix categories are limited to certain screeves. In Colorado, government vehicles affix a "GVT" code vertically before the plate number. Octave positions are sometimes shown by the addition of an affix or small mark. True objects do not stand in cross reference with any affix in the verbal. Currently, astronauts must affix themselves to the surface being worked on.