Hampshire win the race to sign Kent batsman Sam Northeast on a four-year contract.
Australia batsman Shaun Marsh will start the 2018 season for Glamorgan, who have announced a pre-season trip to Dubai.
Derbyshire sign New Zealand international spinner Mitchell Santner as overseas player for the second half of 2018.
England all-rounder Adil Rashid will only play white-ball cricket for Yorkshire during the 2018 season.
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Hampshire focus on securing a T20 franchise from 2020 after missing out on any Test matches between 2020 and 2024.
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How to get into Cricket
How to get into cricket - physical exertion with a chess-like element in matches which range from half an hour to five days.