Westlaw
Within Westlaw, when you go into the Secondary Sources you can further select "Texts and Treatises" - from here you can select topic, jurisdiction and other filtering options.
Lexis Advance
Within Lexis Advance, when you click on Secondary Materials you can further select "Treatises, Practice Guides & Jurisprudence" - from here you can select topic, jurisdiction and other filtering options.
Bloomberg BNA
Within Bloomberg Law, you may search Treatises and other Secondary Sources by topic under Practice Centers.
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Examples of Online Treatises
Moore's Federal Practice - Available Online in LexisNexis Digital Library (Updated March 2022).
Search and Seizure - Available Online at Westlaw (Last Updated December 2021).
Nimmer on Copyright - Available Online at Lexis Advance (Updated December 2021)
Treatise on Constitutional Law - Available online at Westlaw (Updated May 2021).
Gitlin on Divorce - Available Online at Lexis Advance (Updated June 2021)