Ag/AgCl reference electrode is a kind of commonly used reference electrodes.
AgCl + e = Ag + Cl- E0 = 0.199V vs. NHE 25 C
D.J.Ives and G.J.Janz, "Reference Electrodes," Academic Press, New York, 1961

There is an example of adding a Luggin Capillary to reference electrode. The purpose is closing the reference electrode to the surface of the working electrode for reducing the IR drop.

Regarding Luggin Capillary, the figure below is a diagram of the reference electrode was drawn to classical textbooks. There is a salt bridge between the working electrode and the reference electrode. But in this case, the distance from the working electrode and the reference electrode is long that easily courses potential instability. And the air is also easy to enter long salt bridge. In particular, the high frequency impedance measurement will be affected,

Luggin Saturated Silver/Silver Chloride Reference Electrode is integral to the reference electrode and luggin capillary, which was to simplify the structure shown above.
Electrode in a double structure, saturated KCl solution put inside, which is it is easy to refill or change the saturated KCl solution. The liquid junction of the inner tube is porous ceramics. The electrolyte solution is sucked into the outer tube with a rubber dropper.