Only one team will be promoted from Division Two of the County Championship this summer in order to reduce the number of top-tier teams in 2017.
A top division of eight teams will be introduced next year, down from nine, with 10 counties in the second.
Each county will now play 14 games rather than 16, while the Royal London One-Day Cup moves to early season.
"The changes for 2017 will be good for fans, players and our international teams," ECB chairman Colin Graves said.
One-Day Cup group games in April and May ahead of a July final does away with the end-of-season Lord's showpiece, and allows the T20 Blast to take centre stage in the second half of the season.
A two-tier T20 Blast has been rejected, at least until the next broadcast deal in 2019, when plans to change the competition to a city-based franchise tournament could also be revived.
Four-day cricket will again largely take a break during July and August.
"The season is easier to follow," Graves added. "The blocks help players focus on specific skills, and there's a better balance across all three formats."
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