Nottinghamshire pressed home their advantage with the bat - courtesy of a century from Sammy King - before snaring three Northamptonshire wickets to finish in the ascendancy on day two at Lady Bay.
The hosts finished the first day 388/5, with half centuries from Dane Schadendorf, Freddie McCann, Calvin Harrison.
King and Travis Holland added 145 for the sixth wicket, to take the Green and Golds most of the way towards their final total of 482/9d.
With the 21 remaining overs of the day, Jake Ball struck twice to see James Cronie and Gus Miller removed, before Toby Pettman dismissed Arush Suchake, caught at point, to leave Northants 65/3 after James Hayes had struck the top order batter in the helmet with a ferocious bouncer.
Notts will resume on day three with a lead of 417. You can view the live scorecard here...
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The Royal London Cup Final
The timeless pomp, ceremony and tradition of county cricket's historic 50-over final is soon to take place at Trent Bridge.
Lancashire and Kent will go head-to-head in the showpiece finale of the Royal London Cup at our historic home on Saturday 17 September.