The measurements are performed on patients under investigation for AF at cardiology ambulatory Y at Frederiksberg-Bispebjerg Hospital and Aleris-Hamlet Hospital, Copenhagen. The patients are outfitted with a C3 monitor simultaneous with the use of the conventional ambulatory apparatus. This is a "Holter monitor" (Rozinn station RZ152+12 and RZ153+ from ScottCare Holter for Windows 2010 (CE0120) and "EventCare SA version" (CE0120), both bought via SEMCA Ap, Skævinge, Denmark.