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Everyone who has used Rails for a while has their own set of “commonly used” helper methods. Many times, though, those commonly used sets are different. Some people use FormHelper for all of their forms, while others prefer FormTagHelper. Some people use AssetTagHelper, while others handcode links to static resources like stylesheets and images.
Rather than provide a comprehensive reference to these methods—the API documentation does that—this appendix provides a catalog you can browse to decide which methods might actually prove useful to your own needs. Much of the difficulty in using helpers is in finding them before you reinvent the wheel yourself.