Former England captain Alastair Cook made his 71st first-class century as Essex made a solid start against Yorkshire on Thursday, while new Test skipper Ben Stokes featured for Durham at Worcestershire.
Cook compiled his runs against a Yorkshire attack featuring the occasional off-spin of Joe Root, who recently stood down as England captain after succeeding the left-handed opener.
The 37-year-old Cook, who played the last of his 161 Tests in 2018 before retiring from international duty, made 107 from 268 balls.
His third-wicket stand of 174 with Paul Walter guided Essex to 234 for three on the opening day of this First Division match at their headquarters in Chelmsford, east of London.
Elsewhere in the top-flight, Pakistan's Mohammad Amir took three for 51 but Hampshire still posted 310 for eight against Gloucestershire at Southampton, with Felix Organ making 107 after being promoted to opener.
In the Second Division, Stokes looked on from the pavilion at New Road in what was his first appearance since he was confirmed as England's new Test skipper last month.
The all-rounder saw South African Sean Dickson hit 104, his third Championship century of the season, in a total of 339 for three.
Stokes is due to bat at number six.
Australia's Marnus Labuschagne, the world's top-ranked Test batsman, proved the worth of his leg-spin by taking three for 65 as Glamorgan reduced Leicestershire to 285 for eight in Cardiff.