A folding machine is a machine used primarily for the folding of paper. Folding is the sharp-edged bending of paper webs or sheets under pressure at a prepared or unprepared bending point along a straight line according to specified dimensions and folding layouts. Paper can be folded with either a buckle or a knife; thus, there are generally three types of folding machines: buckle folders, knife folders or a combination of these two types. Whilst buckle folding is the more popular of the two methods, knife folding is sometimes preferable. Folding machine models vary in sophistication, with high-end machines capable of processing more complex folding jobs and unusual paper forms (in terms of density and size). Organizations required to undertake mass mail-out campaigns often employ folding machines to improve efficiency. However it is very commonly used finishing process across the printing industry. Buckle folders Buckle folders work by feeding the paper at high speeds until it can move no further; the reaction of the paper is to buckle. High friction rollers then grip the paper and push it through a pair of folding rollers. The paper is squeezed between the rollers in the process which results a fold. Attached rubber provides the rollers with the required grip. The front edge of the paper is then finally placed into a "pocket"—a result of the rollers pressing together with the aid of a spring. Knife folders A knife folder works by striking the paper with a knife between two rollers. This knife is not sharp enough to cut through the paper and simply strikes and creases the paper along the line where the fold is required. Ideally, knife folding is used with thick paper, for "cross folds" (commonly employed for maps and also known as a "French fold") or if the paper has been stitched during the folding process; stitched sheets can ruin high friction rollers. Paper folders Folding machines exclusively used for folding paper are commonly referred to as paper folders. These machines are typically used for creating letter folds (C Folds) and accordion folds (Z Folds). Other commonly used folds include a half fold, fold-out, double parallel fold, gate (brochure) fold, and right angle fold. The fold type in a buckle folder is set by adjusting folding plates. Most paper folders allow for a wide range of fold types. Right angle folds require the paper to rotate 90 degrees also