Bill in Williamsport, Pa., has a question about an odd possessive construction: “Joe is a friend of John’s.

 “A friend of John’s” is something called a double possessive or a double genitive. The word “double” captures the problem Bill laid out: it uses two methods to show possession where only one is needed. “Of” creates possessives. So does an apostrophe plus an S. Here's my recent column that takes a thorough look at what's wrong — and what's right — with double possessives.

Good grammar is important for good writing. But to be honest, that’s usually the easy part.

Ned in Albany had a question about the phrase, used in this column, “that works out great.” He asked, “Isn’t ‘great’ an adjective and what’s it modifying here? Shouldn’t it be ‘well’ in uncorrupted English?”

Well, no. Adverbs are subtler beasts than most of us are taught. We tend to think of them offshoots of adjectives. “Quick” is an adjective whose job is to modify a noun. “Quickly” is its adverb equivalent and its job is to modify verbs.

Often, that works like a formula. Add “ly” to an adjective and you have an adverb. But not always. If you’re fast at typing, you don’t type fastly. If you’re right about how to cut a pineapple, you don’t cut it rightly. If you wear ugly clothes, you don’t dress uglyly.

Here's my recent column about mastering the complexities of adverbs.

 I recently wrote a column pointing out that the practice of putting two spaces between sentences is obsolete, rooted in the days when typewriters gave every character the same amount of space.

Reader Cyndy, a professional typesetter (now referred to as a “desktop publisher”) who studied typesetting in college, had a lot of extra insight.

“When typing on a typewriter was the norm, double spaces indeed made the paragraphs easier to read. But when typesetting on computers came along, the issue of justifying a paragraph became the issue. (No staggered right anymore. It was a smooth line on both vertical sides of the paragraph or story.)

If you put a double space in there, justifying the type may have caused it to be ragged on the left side, because if the sentence ended at the end of the line and there was a double space, then there would be a space at the beginning of the next line, thereby killing the smooth vertical line.

Here's some more good stuff I got from readers recently.

In the pro-wordsmithing realm, error-free prose or nearly error-free prose serves an important purpose.

It says you’re serious about serving the reader. Your form reflects on your content: Careful, meticulous grammar, spelling and punctuation signal that you put great care into your information-gathering, too.

When text isn’t professionally edited, I notice. And obvious grammar errors aren’t the only mistakes that tip me off. Little tells — rules professional editors follow that other people don’t know about — always give the amateur away.

Here are just a few of the tells that can tip your hand.

We editors love to criticize spell-check. We know too well how this tool designed to save you from embarrassing errors can let you down. If you type "Please remain clam," which as I reported in this space a few months ago one unfortunate writer did, spell-check won't know that you wanted to type "calm."

"Stay for awhile" is, technically, a grammar mistake. A preposition like "for" takes as its object a noun phrase — a noun or pronoun with or without modifiers. "For" is a preposition, but "awhile" isn't a noun. It's an adverb. So it can't be the object of "for."

"A while," on the other hand, is a noun phrase. It can be the object of the preposition "for." So "Stay for a while" is correct.

But if you take out the preposition, the dynamic changes, which is why both "Stay awhile" and "Stay a while" are correct. Here's a column I wrote a while back explaining why noun phrases like "a while" can function adverbially even though adverbs like "awhile" can't function as nouns.