From experience the printer knows that many authors get a different impression of what they have written when they see it in type from what they had when they read it in manuscript, and it frequently happens that alterations on proof are very numerous in consequence. When either from this or any other cause numerous alterations are anticipated, the matter is first set up in long slips called "galleys," and not put at once into page form. As soon as a few of those galleys are composed an impression called a "proof" is taken from the types so set, and this proof is passed to a reader whose duty is to see that a correct copy is made of the manuscript, and that the spelling is accurate and the punctuation good. This is a work commanding considerable intelligence and experience, as the number of types required for a printed page is very great, and even the most expert compositor cannot avoid mistakes. This marked proof is returned to the compositor to make the necessary corrections. Fresh proofs are got till no further errors are detected, when a final proof is pulled and sent to the author, who makes such alterations as he may desire. When the corrected proofs are returned by the author they are given to the compositor, who makes the required alterations in the type. After this a revised proof is submitted. When the author is satisfied that the reading is as he wishes he returns the proofs, and the galleys are now made into page form. If it is not expected that the author will make many changes the types are arranged in page shape before any proofs are shown to him, and the work goes through somewhat more quickly. When the types are divided into pages they are placed in sets or "formes," each forme being secured in an iron frame called a "chase," which can be conveniently moved about. Each chase is of a size to enclose as many pages as will cover one side of the sheet of paper to be used in printing. Fifty years ago only one or two sizes of paper were made, and the size of sheet generally used for books was that which allowed eight pages of library size on one side, hence called "octavo" size, or when folded another way allowed twelve pages, hence "twelvemo" or "duodecimo." Other sizes occasionally used are called "sixteenmo" or "sextodecimo," "eighteenmo" or "octodecimo," etc. With larger sized printing machines now driven by steam or electricity, there is greater variety in the size of formes and papers used in printing. In all cases, however, the number of pages laid down for one side of paper must divide by four. The pages are set in the chase in special positions, so that when the sheet is printed on both sides and folded over and over for binding they will appear in proper sequence.