Grammar and style checker programs are coming into style. They've been around for years but are now becoming standard fare for the average writer. They're following spelling checkers as a once-nice, now-necessary utility for word processing.

How can a computer check your grammar and style? Isn't that the sort of subtle judgment that only a human brain can make?

Yes and no. It's true that grammar and style decisions are still firmly in human hands. Computers don't understand all of the subtle variables that influence such a decision. Only a person can intelligently decide which writing techniques to use, which rules to break and which words to choose.

But a computer can tell you which rules have been broken, from clear-cut punctuation rules -- where to put quotation marks -- to more sophisticated grammar rules -- verb-subject agreement -- to abstract style rules -- avoiding the passive voice. Any and all of these rules are breakable and for good reasons in appropriate situations. But most of us break them without noticing, and our writing suffers because of it.

Give the grammar checkers credit. Although they are generally inexpensive utilities, they actually have to pack a lot of artificial intelligence and advanced programming to analyze the contextual meaning of language.

Two of the most popular grammar and style checkers for the Macintosh are Grammatik Mac 2.0 (Reference Software, $99, 1-800-872-9933) and Correct Grammar (Lifetree Software, $79, 415-541-7864). Neither is new, though Grammatik has been around longer. Both showcase the same trend in software -- multiple platform availability. That is, both are also available for the IBM Personal Computer and its clones.

Grammatik has a Mac version, a standard MS-DOS version and a version for Microsoft Corp.'s Windows program. I'm a big fan of programs that play to all of these audiences.

The three versions of Grammatik are nearly identical. The Mac version comes with a free HyperCard stack, "American English Writing Guide." The PC version (Grammatik IV) comes with a book called "Secrets of Successful Writing." This is a fine, brief guidebook to better writing.

The PC and Windows versions come with a Rule Editor program that lets you add your own grammar and style rules to the checking process. The Mac version doesn't have this on the original disk -- although you can get it free when you register your program.

Grammatik Mac works on just about any kind of Macintosh text file. You can tell it to mark all errors and save the marked copy in a new disk file. Or you can run in an interactive mode. Interactive Grammatik immediately shows you each possible error, advises on how to fix it and asks if you want to mark the error, change it or ignore it.

Sometimes the advice is specific -- such as use "that" instead of "which" -- but sometimes the advice is more general: "This sentence is too long. Consider shortening it or breaking it up." You choose which rules to check, turning them on or off as you like, from spelling (you could use Grammatik in place of a spell checker) to particular points of punctuation.

There's context-sensitive help built in to explain any of these rules and the menu commands.

You may tune the checking to the type of document you're examining -- business, technical, fiction, general, informal or custom. If you save these preferences for use later, you'll have various arrows in your grammar quiver for your various writing tasks. The rule editor program does let you add your own rules, although this will require a bit of study on your part to master the editor's syntax.

After examining a document, Grammatik can produce some statistics about your writing. It can calculate several indexes of how difficult your selection is to read, a listing of how often you use various words and an analysis of the average word, sentence and paragraph lengths.

Correct Grammar looks a lot like Grammatik. Most everything I just said about Grammatik also applies to Correct Grammar, from working on a variety of file types, to the interactive option, to selecting which rules to use, to the writing statistics it can report.

Correct Grammar also includes spell checking and gives you advice on fixing grammar and style errors. There is a tutorial and on-line help, and for now, at least, a free "Writer's Guide" vest pocket volume about grammar and style.

Knowing which of the two to choose is difficult. If you get Grammatik bundled with your new copy of the WriteNow word-processing package, you don't have a decision to make. Otherwise, it's mainly a choice on price, interface and grammatical expertise.

Correct Grammar lists for $20 less, but the "street" prices don't differ by that much. The two programs have similar, but not identical, sets of windows and buttons, and you may like one more than the other. I slightly prefer Correct Grammar on this count.

That leaves grammatical expertise -- which catches more errors? I've seen several analyses of this question, and the answer seems to be: Each catches some errors the other doesn't. My feeling about this point is that unless you're a fantastic writer, don't worry too much about either program missing a problem here or there. Either will catch enough errors to keep you busy improving for some time.

I'd like to see some improvements in both programs. It would be nice if they could pop up as desk accessories or memory-resident programs on a PC so you could analyze text in any program: presentation package, data-base report or word processor. I'd also like to analyze a paragraph at a time, instead of the full document. And I'd like to compare two drafts of a document directly for the improvement in grammar and style.

My conclusion is simple: Get a grammar and style checker. Correct Grammar is a bit cheaper and has a slightly more comfortable interface. Grammatik is available for more computers and has an advanced rule editor for customizing checking to your own or your company's style. Both Grammatik and Correct Grammar can teach you about grammar and style and improve your writing.

Phillip Robinson is an author of books and articles about computers and an editor for Virtual Information of Sausalito, Calif.