These involve “training” the program to recognize errors by providing it with a large number of examples. More advanced functions may rely on machine learning techniques. The diagram below illustrates how a paper checker handles a simple task: spell-checking. They may catch simple grammatical errors via similar rules: a rule that periods must precede (and not follow) an ending quotation mark, for example, would help the app catch improperly-formatted quotations. Any word that does not match a word in the database can be quickly flagged as misspelled. For example, they may catch spelling errors by comparing each word in the paper to a large database of recognized words. Paper checkers’ simpler functions usually operate according to rote systems of rules. Some apps perform both very simple functions (like checking for spelling errors) and complex functions (like looking for sentence structure errors) as they review your paper. Like most digital products, paper checker apps vary greatly in their quality, reliability, and sophistication. For this reason, the “best practice” guidance in this resource is intended to help you exercise your judgment if you choose to use a paper checker app. In short, like spell checkers, paper checkers work best when their advice is secondary to the author's own judgment. Used carelessly, however, they can become crutches that narrow writers' horizons, prevent true growth, and even sometimes introduce new errors. Some apps even boast the ability to pinpoint tricky voice and style concerns (like, for instance, inappropriate use of the passive voice) and give on-the-fly suggestions for improvement. These powerful new features make the author's oversight even more important. Used judiciously, paper checkers can make the work of revision quicker and easier than ever before. Most at least provide sentence-by-sentence grammar feedback. Recently, “paper checker” applications that tout features far beyond basic spell checking have emerged. Most writers simply understand that the editorial input a spell checker provides must be secondary the author's own judgment. They are such handy labor-saving tools that we now tend to take them for granted, despite the fact that they can occasionally make mistakes (for instance, by flagging a foreign word as improperly-spelled). Spell checkers make life easier for writers by reducing the time they need to spend re-reading their work for typos. If you’ve used a word processor in the past several decades, you are probably already very familiar with the idea of a “spell checker.” This, of course, is a feature of a program that notifies you when you type a misspelled word. See also our similar article on citation generators. This resource gives a general sense of how so-called “paper checker” applications work and provides best practices for their use.