In Memory of Tony Krueger, the Man Behind Word's Red and Green Squiggles

Who Tony Krueger was

Raymond Chen's blog The Old New Thing published a memorial on June 22, 2026, for a Microsoft engineer whose work almost everyone has seen but whose name almost nobody knows. Tony Krueger died unexpectedly on April 14, 2026, at the age of 58, according to a comment from his father under the post. He graduated from Stanford in 1989 and joined Microsoft immediately afterwards, having already spent two summer internships there, and was close to completing his 37th year as a full-time employee when he died.

Wikipedia remembers Krueger as the person who ported the game Chip's Challenge to Windows for the Windows Entertainment Pack. According to Chen, he did that without access to the source code: he reverse-engineered the MS-DOS version and reimplemented it for Windows. But that is probably not the code that touched the most people.

From a blocking spell check to red and green squiggles

Krueger worked on Word 1.0, 1.1 and 2.0, then on Word for OS/2 and Word for Mac, and later returned for Word 6.0 and several versions beyond it — a run Chen calls a likely record for the most versions of Word shipped by one person.

In the early versions of Word, spell checking was something the user explicitly invoked and then waited for: the program searched the document and presented potentially misspelled words one at a time. Word later added an Auto Spell Check that ran while the user was idle, but it was still a blocking operation, and many users turned it off because it always seemed to start just as they wanted to save and exit.

Krueger reworked the spell checker so that it no longer interfered with foreground work. When it found a problem, instead of waiting to be triggered, it immediately drew red squiggles under potentially misspelled words — and later green squiggles under possible grammatical errors.

Penn and Teller, "Weird Al" and a legacy in every word processor

Krueger was an early fan of the comedy duo Penn and Teller. A friend and colleague attended a show and asked them to sign a photograph for him, explaining: "He was on the team that did the red and green squiggles in Word." Penn Jillette, Chen writes, announced in a voice that filled the theater: "The red and green squiggles!? I love the red and green squiggles!" Teller silently agreed. The autographed photo arrived for Krueger's birthday, and it was not clear whether he was happier about the photo or about the duo's reaction to his feature. Years later, when "Weird Al" Yankovic released the parody video "Word Crimes", in which the red squiggles make a brief appearance, the same friend got him to sign a screenshot.

Today red, green and blue squiggles appear in nearly every word processor, and often outside them. As Chen puts it, Krueger did it first.