These stamps commonly are marked in some way that invalidates their unauthorized postal use. Overprints may be handstamped using inked rubber or metal dies, or the stamp sheets may be put back onto printing presses to receive the added inscriptions. Overprints may be used to commemorate special events, to note changes in governments, to limit or expand the useage of stamps , to denote stamps produced for publicity needs, as well as for many other purposes. This example notes the reversion of an oil concession to the government. In this case, however, the purpose is to limit useage to bulk-rate mail.
Most coils have perforations only on two sides and straight edges on the other two sides.
Bisects are not legal in most countries but may be tolerated or even encouraged in times of crisis when the full range of needed stamp denominations are not readily available. Bisects obviously are only collected on cover because anyone can cut a stamp in half at any time. The Bolivian petrostamp above was bisected in 1956. Some booklets like the one above, may have oil themes or advertisments on the covers or on paper pages between the stamp sheets. Booklet stamps commonly have one to three straight edges that distinguish them from sheet stamps.