What is a RJ9 connector and a RJ9 jack?

RJ9 connector is a modular 4P4C connector. RJ9 connector is normally used for telephone handset and telephone headset. It looks like the regular telephone cord connector RJ11, but slightly smaller. The width of RJ9 connector is 7.7mm while the telephone cord connector is 9.65mm. (Please see picture below.)

RJ9 jack is the jack for RJ9 connector. It is used on telephone to connect telephone handset and telephone headset. (Please see picture below.)