In many ethers, the atoms are arranged in a closed ring. Cyclic ethers which have 3 atoms forming a ring (one oxygen atom, two carbon atoms) are called epoxides or oxiranes. These are also sometimes referred to as alkene oxides. Shown below are some examples: Cyclic ethers which have 4 atoms in a ring (one oxygen, three carbons) are called oxetanes (See picture below). Most other cyclic ethers are named systematically using IUPAC nomenclature rules, although there are a few with frequently used trivial names such as tetrahydrofuran, THF, (See picture below). Large rings which contain multiple ether units constitute a class of materials called crown ethers. These are commonly named using the notation "x-crown-y" where x = the number of atoms forming the ring and y is the number of oxygen atoms. MSDS Relevance: Again, the risk of peroxide formation is particularly high with ethers. Be sure to read about peroxides so you know how to detect, avoid and/or deal with them should you encounter them. Many ethers (such as diethyl ether) are highly volatile and flammable. Many ethers, particularly the crowns, are eye and skin irritants and/or toxic. Always read the MSDS before using an ether so you know what safety precautions to take. Use appropriate engineering controls, wear appropriate personal protective equipment (PPE), have good ventilation, and avoid heat or ignition sources. See also: Alcohol, Peroxides