A red-letter day for the stock market brings us this week’s Marketplace Money buzzword: “M.”

The letter “M” is a ticket symbol on the New Yrok Stock Exchange. It was assigned to Macy’s Inc. this week.

That’s a big deal for two reasons. Exhibit A: companies covet one-letter symbols. Cause well, there’s only 26.

Exhibit B: Most analysts assumed the “M” symbol would go to Microsoft — if the software giant ever listed on the New York Exchange. Right now the company lists on the NASDAQ, where most companies have four-letter symbols.

Maybe Microsoft will get a new one for itself: D-A-R-N.