Although ethers are usually linear molecules, there is no reason for not making circular ones. These are the crown ethers - joined head to tail and with many linking oxygens. The first one produced was 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-diene : quite a mouthful to say. So they were renamed as 'crown ethers' - the origional becomes merely dibenzo[18]crown-6 with '18' indicating the number of atoms in the macrocyclic ring and the '6' referring to the number of those atoms that are ether oxygens.

They are multidentate liganding molecules that are very effective at complexing metal ions. They are also quite sensitive to the size of the metal ion guest, and can be used to discriminate metals by this mechanism. The selectivity comes from the structure of the metal-ether complex, which can be distorted or flat. A [18]crown-6.Potassium complex is fully flat, and binds much better than with the smaller sodium or larger caesium ions.

They have been used, attached to a solid support, to filter out unwanted metal ions from a mixture - purifying it. This can be useful for extraction of valuable or toxic metals from solutions. The ability to bind specific metals also makes them potentially useful for molecular switches.