"Jar", known as "Pod" (ポッド Poddo) or "Pot" (ポット Potto) in the OCG, are a series of low-Level Flip Effect Monsters that help the controller and/or the opponent gain field and hand advantage. Many of the Flip Effect "Jars" have been banned or Limited because of their effects.
Cyber Jar and Fiber Jar for a long period of time were known as the best flip effect monsters playable in the game. Morphing Jar, however, unlike its counter parts is Limited to 1 per deck rather than being banned. This is because of the OTK/FTK deck known as Empty Jar. This deck constantly flips Morphing Jar face-up and face-down using Book cards to discard entire hands and redraw 5 for each player. Repeating this until your opponent has 0 cards in their deck so they may not Draw means you win by Deck Out. Fiber Jar however, is still to this day considered the best Jar card, as it quite literally restarts the entire duel.