Was Jeconiah the son or grandson of Josiah?

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He was Josiah's son.

And Josias begat Jechonias. Matthew 1:11

He was Josiah's grandson.

And the sons of Josiah were, the firstborn Johanan, the second Jehoiakim, the third Zedekiah, the fourth Shallum. And the sons of Jehoiakim: Jeconiah his son. 1 Chronicles 3:15-16