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.