Most Holter Monitors monitor the ECG via two or three channels. Depending on manufacturer, different lead systems and numbers of leads are used; the number of leads may be minimised for patient comfort. Two or three channel recording has been used for a long time in the Holter monitoring history; 12-channel Holters were introduced later, using either the standard 12-lead electrocardiograph or the modified (Mason-Likar) exercise lead system.

These Holter monitors can occasionally