Bach's Fugue no. 2 in C Minor from The Well-Tempered Clavier, Book I, BWV 847, showing the subject, answer, and countersubject (Benward & Saker 2009, 2:57).

Subject and CounterSubject: Some Notes on Barthes and Schumann
Subject and Counter-Subject Detection for Analysis of the Well-Tempered Clavier Fugues
Subject and Counter-Subject Detection for Analysis of the Well-Tempered Clavier Fugues
Subject and Counter-Subject
Regular Countersubjects in Fugue
Bach's Use of Hitherto Unrecognized Types of Countersubjects in the "Art of Fugue"
Failure Analysis of a Multi-Module Counter Subject to Electrostatic Discharge Damage
Personnel scheduling at check-in counters subject to stochastic demand
Governing Subject Vs. Artistic Counter-Subject
From: OAL Front Counter Subject: RETURN OF APPROVED RULEMAKING MATERIALS