Coil Stamp

A coil stamp is a stamp produced by winding a continuous strip of printed stamp paper into a roll, designed for use in vending machines and affixing equipment. Unlike sheet stamps — which come in flat rectangular panes with perforations on all four sides — coil stamps are perforated only on their two “inner” edges (the top and bottom if the roll feeds horizontally, or the left and right if it feeds vertically). The outer edges, where the strip was cut from the paper web during production, are straight and unperforated.

How Coil Stamps Are Produced

Modern coil stamps are typically printed by rotary offset or gravure presses on a continuous web of stamp paper, which is then perforated in strips and wound onto rolls. Depending on the press configuration, the stamps may be oriented “horizontally” in the coil (each stamp is wider than it is tall, as seen when peeling from the roll) or “vertically” (taller than wide). The feed direction determines which two sides carry perforations.

Early coil stamps, particularly US issues from 1908 onward, were produced by converting flat sheet stamps: sheets were cut into strips and the edges trimmed to make them suitable for vending machine loading. These “converted” coils are historically important and may differ from contemporary sheet stamps in subtle ways.

Identifying Characteristics

The definitive feature of a coil stamp is straight edges on two opposite sides — but this identification is not foolproof. Sheet stamps can also have straight edges if they came from the margin of the sheet and the perforations on one or two sides were not printed. The key is to identify which two sides are straight (opposite each other, not adjacent) and to confirm that the straight edge is genuinely unperforated rather than the result of trimming perforations.

Additional distinguishing features often include slightly different gum types (coil stamps may use different adhesive formulations suitable for vending machine dispensing), different paper fluorescence levels in some eras, and occasionally different perforation gauges from the sheet versions of the same design.

Line Pairs and Plate Number Coil Strips

Line pairs are a US-specific variety. When printing plates met end-to-end in early US coil production, a visible line appeared on the paper between the two plates. A “line pair” is a se-tenant pair of coil stamps with this join line running between them. Line pairs from the pre-1920 era are among the most keenly collected US classic issues.

Plate number coil (PNC) strips replaced line pairs as the dominant US coil specialty. From 1981 onward, USPS coil stamps included small plate numbers printed at intervals in the sheet — typically every 52nd stamp. A strip of five coil stamps with the plate number stamp at the centre is called a “strip of five” PNC. Specialist collectors assemble full sets of all plate numbers for each issue, and some plate-number combinations are scarce. PNC collecting is a major US philatelic specialty with its own organisations and publications.

Distinguishing Coils from Trimmed Sheet Stamps

The distinction between a genuine coil stamp and a sheet stamp from which perforations have been trimmed is important: trimming perforations is a form of alteration that misrepresents the stamp’s identity. Techniques for distinguishing them include comparing the stamp’s dimensions (genuine coil stamps may have slightly different design-to-edge spacing than sheet versions), examining the straight edges under magnification for signs of trimming (traces of perforation stubs or irregular cut lines), and comparing paper and gum characteristics.

Storage Considerations

Coil stamps are often collected in strips of three, four, or five to demonstrate their coil format clearly — a single coil stamp without surrounding stamps is harder to identify definitively. Strips should be stored in appropriately sized stamp mounts cut from strip mount material. Longer strips (particularly five-stamp PNC strips) need wide horizontal mounts and should be handled carefully to avoid stressing the perforations between stamps.

Measuring coil stamps with a perforation gauge and comparing to known sheet versions of the same design is essential for building a technically accurate collection.