A copy set is a stack of sheets of paper, usually between two and six in number. The sheets are treated so that when a person writes on the top sheet, the same writing appears on all the sheets; the effect is also achieved when the copy set is run through an impact printer. The sets are called carbonless to distinguish them from the copy sets of decades ago which, due to limited technology, required carbon paper. Copy sets are used most widely for preprinted forms where a human fills in the blanks.

Within a typical carbonless copy set:

• On the front of each sheet except the first is a coating of resin.
• On the back of each sheet except the last is a coating of capsules containing a dye.

• Adhesion. When a suitable cement is applied to the edge of the set, it penetrates a short distance, melting the back and front coatings of adjacent sheets. This forms a sticky goo which holds the edges of the sheets together. The last part of one set will not adhere to the first part of the next set, because the coatings are not present.
• Image Formation. After assembly of the set, when a person writes on the first part, the force of the pen breaks open the capsules on the back of one sheet releasing the dye which mixes with the resin coating on the front of the next sheet. As the chemicals combine, their product turns a black or blue color, producing an image. Full darkness requires several minutes.

By "suitable cement" is meant not ordinary padding glue, which would cause all sheets to stick together, but rather a proprietary solvent. Manufacturers differ in their choices of chemicals for the resin and dye capsules, so one manufacturers' cement may not work with another's paper. Similarly, mixing different brands of paper within a set may not work, as one company's dye may not react with another company's resin.

The configurations of coatings are designated thus:

• The first part is CB: Coated on the Back.
• Intermediate parts are CFB: Coated on the on the Front and Back.
• The last part is CF: Coated on the Front.
• A variant is SC: Self-Contained.

The paper stock chosen is usually a bond of basis weight between 15 and 25 pounds. Thinner paper gives clearer images and requires less pressure in handwriting; thicker stock jams less in the printing process.

Typical Configuration. Below is a cross-section diagram of an ordinary four-part carbonless set. The edge of each sheet of paper is shown in its traditional color (white for the original, with copies in canary, pink and gold). The heavy black line on the top of some sheets represents the resin coating, and the black dots on the bottom of some sheets stand for the dye capsules — but in reality the coatings are transparent. The front of the white sheet (in the diagram, the top surface) is the usual location for handwriting, although handwriting also works satisfactorily on the fronts of the other sheets.

1st Part CB
2nd Part CFB
3rd Part CFB
4th Part CF

Xerographic or offset printing is routinely performed on the front of all sheets. However, the ink if applied to the back of a sheet will interfere with the dye capsules.

Self-Containment. The dye capsules appearing on the back of the sheet can cause a ball-point pen to skip. Hence self-contained (SC) carbonless was introduced to allow a copy to be made from a plain-paper original. In the case of a bank check, there are no capsules on the back to make endorsement difficult.

1st Part Plain
2nd Part SC

If an SC sheet is crumpled, images will develop in the creases because the dye capsules have been broken and the necessary resin is nearby.

The SC coating is almost always applied to the front of a sheet, as there are few cases where it is useful on the back. However, there is nothing wrong with having both the self-contained coating on the front of the sheet and the dye capsules on the back, as in this three-part set:

1st Part Plain
2nd Part SC-CB
3rd Part CF

Packaging. The best-known standard quantity of paper is the ream, 500 sheets. Manufacturers aim to to wrap precollated copy sets by the ream, but approximations are sometimes necessary because the numbers 3 and 6 do not divide evenly into 500. Here are the usual quantities:

Parts per set	Sets per wrapper	Sheets per wrapper
2	250	500
3	167	501
4	125	500
5	100	500
6	84	504

Collation. Some printing machines invert the sheets as they are being handled; others do not. To accommodate different equipment, manufacturers offer two collation schemes for precollated sets. With the three-part version as an example, here are the two sequences of factory packaging:

	Forward (or Straight) Collation	Reverse Collation
1st Part CB			3rd Part CF
2nd Part CFB			2nd Part CFB
3rd Part CF			1st Part CB
repeat			repeat

It is crucial that the sets be forward collated at the time of gluing. Otherwise some sheets will not adhere, and those that do adhere will not form correct sets.

For two-part sets the forward and reverse collations happen to be identical.

Standard Colors. These date back a number of decades, well into the era of carbon paper:

		Color of Each Part
		First	Second	Third	Fourth	Fifth	Sixth
Number of Parts in Set	2	White	Canary
	3	White	Canary	Pink
	4	White	Canary	Pink	Gold
	5	White	Green	Canary	Pink	Gold
	6	White	Blue	Green	Canary	Pink	Gold

All six of these colors are sold uncollated in each of CF, CB and CFB for larger sets or custom color combinations. Also, CF index stock in 90- and 100-pound weights is available in white and manila colors to give the set a stiff backing. Safety CB paper is available for bank checks. Ledger carbonless is sold in the traditional accountants' colors of white, buff and greentint.

Factory-collated sets are also sold with all sheets white.

Carbon Copy Sets. Old-time carbon copy sets often used manifold paper, a thin bond paper ranging from 7 to 11 pounds, along with carbon paper of a similar weight. With two layers of paper (one plain, one carbon) for each copy, such light stocks were required to allow the force from a pen to pass through.