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They don't incorporate the nitrogen at the pouring step, Guinness (and other stouts and ales) are force 'carbonated', stored and dispensed under a 75% nitrogen/25% CO2 blend, whereas most draughts use just CO2.

Guinness requires a different faucet to most draughts, with a restrictor plate that forms the creamy head. The plastic widget in cans of draught Guinness performs the same function as the restrictor plate.

The purpose of all of this is to replicate the head and mouthfeel of a 'real ale' that has been naturally carbonated and dispensed at atmospheric pressure with a handpump.