This set of Headphones is used with Long Distance Microphone System.The headphones have a 3.5mm mono jack and a volume control for each ear.
They can also be used with the Viking Longplay Recorders to monitor the input you are recording (see the Monitor Input Option*).
*A Mono 3.5mm to 1/4" adapter jack is needed to use the headphones with the 1/4" jacks on the recorders.