Glamorgan 452 for 9 (Root 229) lead Northamptonshire 209 (Rossington 77, de Lange 4-64) by 243 runs
Billy Root made the highest score by a Glamorgan batsman against Northamptonshire to put his side in complete command at Wantage Road. Root's career-best 229 helped his side recover from 120 for 5 to close the second day 452 for 9, leading by 243.
Badly dropped by Ricardo Vasconcelos at second slip on 0 - a chance that would have left Glamorgan 21 for 4 - Root took full advantage, hitting 30 fours and a six in his third innings over a hundred in the County Championship and second against Northants this season after his 126 in Cardiff back in April.
He played a number of eye-catching cuts and drives, the pick of which was crashed through cover point off Nathan Buck, and ran superbly between the wickets with one exception - a sharp single attempted to Buck at mid-on would have seen Root run out for 90 had Buck thrown down the stumps, but the fielder failed to gather.
It allowed Root to go through to a fourth first-class hundred in 121 balls with 16 fours and after tea he extended it to a double-century in 230 deliveries with 26 fours and a scooped six over backward-square off Ben Sanderson.
He initially battled Glamorgan away from danger in a stand of 83 with Charlie Hemphrey that took up most of the morning session before adding another 87 for the sixth wicket in the afternoon put Glamorgan in the lead.
Dan Douthwaite was the second game partner for Root in making 44. He produced a beautiful high elbow to strike three on-drives for four before hoicking Rob Keogh's off-spin over midwicket and top-edging a hook at Luke Wood for another boundary as 172 runs were added between lunch and tea.
Douthwaite flicked Keogh to short-midwicket but was only the second afternoon wicket to fall after Kieran Carlson fell for 7 to the ninth ball of the session, edging a lifting delivery from Buck to second slip. At 120 for 5, Northants had hopes of getting back in the game, only for Root to take it away from them.
The home side made the perfect start to the day with three quick wickets for Sanderson. He made the breakthrough with the 13th delivery of the morning with one that nipped away to feather the edge of Nick Selman, who fell to the first ball he'd received on day two.
Sanderson then angled balls back into the right-handed Marnus Labuschagne and David Lloyd, who both shouldered arms and both lost their off stumps within 11 balls of each other. Had Root then been caught, Northants would have been right in the match but the miss proved the worst sliding doors moment imaginable for the hosts.