Leniency. Mr Gould calls this word and lenience "two philological abortions," Lenity is undoubtedly the proper word to use, both Webster and Worcester do recognize ]i]leniency[/i] and lenience.
[Alfred Ayres The Verbalist: A Manual Devoted to Brief Discussions of the Right and the Wrong Use of Words, and to Some Other Matters of Interest to Those Who Would Speak and Write with Propriety, New York, 1895.]
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lenience, -cy. The second is the recommended; see -ce, -cy.
[H. W. Fowler A Dictionary of Modern English Usage, 1st edition, 1926.]
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lenience, -cy. An incipient differentiation, which deserves encouragement, would use -ce for an action and -cy for a disposition. See -ce, -cy.
[H. W. Fowler A Dictionary of Modern English Usage, 2nd edition, revised by Sir Ernest Gower, OUP, 1965.]
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leniency; lenience. The latter is a needless variant—an all too common one. E.g., "Shas Party leader Aryeh Deri ... is alleged to have demanded Bar-On's appointment of lenience [read leniency] in his own trial on charges of fraud in his party's finances." Barton Gellman, "Netanyahu's Indictment Sought," Wash. Post, 17 Apr. 1997, at A1.
[Bryan A. Gardner A Dictionary of Modern American Usage, OUP, 1998.]