After the market's terrible day, the devil's in the quarter-point detail

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Whether the market's thrashing on Friday was sharp or devilish comes down to a quarter point.

The Dow Jones industrial average ended its first tumultuous week in a year by suffering a pullback of more than 2.5 percent. It closed down 665.75 points.

Cue all the headlines and splashy photos with devil references by market watchers who rounded the closing figure up to 666.

Even CNBC went with the 666 figure after some internal debate.

So, what's in a quarter of a point? That sliver of rounding, seemingly minuscule in a market that topped 26,000 just last month, actually means a lot in the age of electronic trading.

Computers and preset algorithms are running lots of trades, and many are built to react when the market hits specific levels. So different elaborate mousetrap actions kick in at 665.75 than might be triggered at 666.

As CNBC markets writer Fred Imbert will tell you as he's trying not to blink while watching the Dow when it's up 99.9 points, he can't change his markets headline to reference 100 points until it's official — no rounding allowed.

This time, though, the temptation of headlining a market gutting of — nearly — 666 points was too much to resist.

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