Merriam-Webster OnLine
Until the O.E.D. is offered online free of charge, Merriam-Webster is the place to go. Besides comprehensive definitions and hyperlinked synonyms, there are nice etymologies and pronunciation guides (with audio). Great for connotation and denotation lessons.
  Merriam-Webster OnLine

Guide to Grammar and Style
Jack Lynch appears twice in this guide for good reason -- quality stuff! This grammar guide is accessible to high school students, comprehensive, and well organized. The introduction alone would spark a debate on language and usage among the prescriptivists and descriptivists in your classroom.
  Guide to Grammar and Style

Strunk's The Elements of Style (1918 edition)
Though you will have to buy the latest edition of this classic style book (The Elements of Style, 4th edition), it is worth knowing that you can find the 1918 edition right on the Web. It provides a short, straightforward guide to writing clear, precise English prose.
  Strunk's The Elements of Style (1918 edition)

Classical Rhetoric

A Glossary of Rhetorical Terms with Examples
From "alliteration" to "zeugma," ninety terms are defined (with examples).
  A Glossary of Rhetorical Terms with Examples

Silva Rhetoricae: The Forest of Rhetoric
This is an award-winning, comprehensive site by Gideon O. Burton (Brigham Young University). The entire site is built on the forest/trees metaphor, taking into account the "big picture" of the purposes and patterns of rhetoric while giving attention to the details of the terminology of rhetorical figures. This is a great starting point for classical rhetoric studies.
  Silva Rhetoricae: The Forest of Rhetoric

Aristotle's Rhetoric
Billed as a "hypertextual" version of W. Rhys Roberts' 1954 translation, this scholarly reference was compiled by Lee Honeycutt (Iowa State University).
  Aristotle's Rhetoric

Writing and Composition Resources

The Nuts and Bolts of College Writing
This fantastic resource addresses writing across the curriculum and writing as a process. Michael Harvey's approach and style should be well received by high school students -- comprehensive and accessible. Harvey includes many examples for revision considerations. Great resource for research paper formats.
  The Nuts and Bolts of College Writing

Paradigm Online Writing Assistant
For students who don't know where to start, this resource leads writers through the process with accompanying exercises and graphic organizers. Special consideration is given to argumentative and exploratory essay writing.
  Paradigm Online Writing Assistant

UW-Madison Writing Center Writer's Handbook
A well-organized handbook-style site that can be of help to students writing in many different modes, from lab reports to literary analysis. The design of this site is very straightforward and easy to navigate.
  UW-Madison Writing Center Writer's Handbook

Purdue University's Online Writing Lab (OWL)
The resources and links at this site are well worth looking at, even for non-Purdue students. A whole section of the site is devoted to Internet resources for teachers. The site also includes a very good tutorial on searching the Internet for research and other resources.
  Purdue University's Online Writing Lab (OWL)

Online Journals and Publications

KAIROS: A Journal of Rhetoric, Technology, and Pedagogy
An online journal that explores writing and instruction in a more technologically advanced world. The focus is heavy on the idea of publishing Web texts and other cyber-media.
  KAIROS: A Journal of Rhetoric, Technology, and Pedagogy

Enculturation
A provocative online journal that might be a little difficult for students, but is rich in ideas for thematic units and arguments. Articles explore issues in writing, language, and culture.
  Enculturation

The Atlantic Online
The Atlantic has set the standard for excellent writing since the mid-1800s. This is a great resource for language discussion -- there's a section devoted to it. Do a search on nearly any topic/theme and you are bound to find something that is not only applicable, but of high literary quality. Use articles for class discussion of use and effectiveness of rhetorical methods.
  The Atlantic Online

The Writing Instructor
Check the TWI Features, Topical Areas, and Resources sections for articles, lesson plans, and ideas for teaching effective writing.
  The Writing Instructor

Print and Electronic Publications for Rhetoric and Composition Scholars
Not a publication itself, but a list of journals compiled by Leah Zuidema (Michigan State University).
  Print and Electronic Publications for Rhetoric and Composition Scholars

Unit Starting Points

Persuasion Analysis
Advertising might be the most intrusive form of rhetoric. Hugh Rank provides a nice resource that can be used as a starting point on the persuasive tactics that are encountered every day in a consumer society. Rank also explores political rhetoric.
  Persuasion Analysis

Great American Speeches: 80 Years of Political Oratory (PBS)
These twentieth century speeches might make for interesting analysis around October and early November. Nice background information is provided along with some short audio and video clips. Speeches are arranged on a historical timeline for context.
  Great American Speeches: 80 Years of Political Oratory (PBS)

Other Resource Guides

Rhetoric and Composition (rhetcomp.com)
This catalogued guide site is geared more for college instructors, but there is much that is useful for AP teachers. "Listserves" and online journal lists are especially good here.
  Rhetoric and Composition (rhetcomp.com)

Mantex Information Design
Even though this is a site for a British book distribution company, take a look at the free downloads and guides that are linked to from the home page.
  Mantex Information Design

Resources for Writing and Writing Instructors
Another great resource by Jack Lynch. This site is a good first stop for anything you may be looking for, with many links to choose from.
  Resources for Writing and Writing Instructors

Back