An interrogatory is "[a] written question (usu. in a set of questions) submitted to an opposing party in a lawsuit as part of discovery." Interrogatory, Black's Law Dictionary (11th ed. 2019).
This guide includes a sampling of the materials Jenkins has on interrogatories. For more sources, try searching the Jenkins' catalog for interrogat*. The search results can be limited by adding additional terms to the search or by modifying the search to a specific library location, i.e. General Collection or Pennsylvania Collection.
Court rules often outline how evidence can be discovered and admitted in court as well as how discovery and interrogatories can be conducted.
The United States Courts website links to PDFs of the Rules of Appellate Procedure, Rules of Bankruptcy Procedure, Rules of Civil Procedure, Rules of Criminal Procedure, and Rules of Evidence. For additional ways to access the Federal Court Rules, including in print, and on the library's Lexis, Westlaw, and Bloomberg Law computers, see the Federal Court Rules guide.
The Pennsylvania Court Rules are available for free online in the Pennsylvania Code (see Titles 210 through 246). For additional places to find Pennsylvania Court Rules, including in print and on the library's Lexis, Westlaw, and Bloomberg Law computers, see the Pennsylvania Court Rules - State guide.
For how to access county court rules, see the Pennsylvania Court Rules - County guide.
Jenkins also has court rules for all 50 states and the District of Columbia. A search of the Jenkins' catalog for court rules and the jurisdiction should find the relevant materials.