A flagon is a large leather, metal, glass or ceramic vessel, used for drink, whether this be water, ale, or another liquid. A flagon is typically of about 2 imperial pints (1.1 l) in volume, and it has either a handle or (more usually) one or two rings at the neck. Sometimes the neck has a large flange at the top rather than rings. The neck itself may or may not be formed into one, two or three spouts. The name comes from the same origin as the word "flask"

The purpose of the rings is that you put your first (or first two) fingers into these or, if no rings, under the flange, or, if no flange, your hand into the handle, palm upwards, you rotate your forearm both up and away, turning your wrist so as to rest the flagon on top of your crooked elbow raised nearly to the horizontal, and you can then turn your head and drink straight from the flagon by pulling down with your fingers and lifting your elbow.

The weight of the flagon is taken by the upper arm, and the control (by flexing shoulder and fingers) is very fine. The flagon was developed for a specific purpose: to take liquid refreshment (usually cider) out into the fields to sustain the agricultural labourers or perhaps, in Roman times, to refresh the rowers aboard ships.

More Info: en.wikipedia.org