Rebecca Loudon mentioned in a recent post that she had learned a new word.  She and I were born in the same month of the same year, scant days apart, and I sometimes feel like our lives rhyme.  But I digress...  Just before reading Rebecca's post I had, myself, learned a new word: "chirality."  It's the title of the first poem in Rae Armantrout's latest collection, Itself.

Here's the Wikipedia definition of chirality:

Chirality /kaɪˈrælɪtiː/ is a property of asymmetry important in several branches of science. The word chirality is derived from the Greek, χειρ(kheir), "hand", a familiar chiral object.

An object or a system is chiral if it is distinguishable from its mirror image; that is, it cannot be superposed onto it. Conversely, a mirror image of an achiral object, such as a sphere, cannot be distinguished from the object. A chiral object and its mirror image are calledenantiomorphs (Greek opposite forms) or, when referring to molecules, enantiomers. A non-chiral object is called achiral (sometimes alsoamphichiral) and can be superposed on its mirror image.

The term was first used by Lord Kelvin in 1893 in the second Robert Boyle Lecture at the Oxford University Junior Scientific Club which was published in 1894:

I call any geometrical figure, or group of points, 'chiral', and say that it has chirality if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself.^[1]

Human hands are perhaps the most universally recognized example of chirality: The left hand is a non-superimposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide.^[2] This difference in symmetry becomes obvious if someone attempts to shake the right hand of a person using his left hand, or if a left-handed glove is placed on a right hand. In mathematics chirality is the property of a figure that is not identical to its mirror image.

Interestingly enough, while I did not know this word, the concept of "nonsuperimposable mirror images" is something that figures importantly in Appearances, that beast of a book I've been working on for the last few years.