Ethers (/ˈiːθər/) are a class of organic compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups—of general formula R–O–R′. These ethers can again be classified into two varieties: if the alkyl groups are the same on both sides of the oxygen atom, then it is a simple or symmetrical ether, whereas if they are different the ethers are called mixed or unsymmetrical ethers A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether" (CH₃–CH₂–O–CH₂–CH₃). Ethers are common in organic chemistry and pervasive in biochemistry, as they are common linkages in carbohydrates and lignin.