Etymology
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Pleiades (n.)

late 14c., Pliades, "visible open star cluster in the constellation Taurus," in Greek mythology they represent the seven daughters of Atlas and Pleione, transformed by Zeus into seven stars. From Latin Pleiades, from Greek Pleiades (singular Plēias), perhaps literally "constellation of the doves" from a shortened form of peleiades, plural of peleias "dove" (from PIE root *pel- "dark-colored, gray"). Or perhaps from plein "to sail," because the season of navigation begins with their heliacal rising.

Old English had the name from Latin as Pliade. The star cluster is mentioned by Hesiod (pre-700 B.C.E.); only six now are visible to most people; on a clear night a good eye can see nine (in 1579, well before the invention of the telescope, the German astronomer Michael Moestlin (1550-1631) correctly drew 11 Pleiades stars); telescopes reveal at least 500. Hence French pleiade, used for a meeting or grouping of seven persons.

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J 

10th letter of the English alphabet, pronounced "jay," as in "kay" for -k-, but formerly written out as jy, rhyming with -i- and corresponding to French ji.

One of the most stable English letters (it has almost always the same sound), it is a latecomer to the alphabet and originally had no sound value. The letter itself began as a scribal modification of Roman -i- in continental Medieval Latin. The scribes added a "hook" to small -i-, especially in the final position in a word or roman numeral, to distinguish it from the strokes of other letters. The dot on the -i- (and thus the -j-) and the capitalization of the pronoun I are other solutions to the same problems.

In English, -j- was used as a roman numeral throughout Middle English, but the letter -y- was used to spell words ending an "i" sound, so -j- was not needed to represent a sound. Instead, it was introduced into English c. 1600-1640 to take up the consonantal sound that had evolved from the Roman i- since Late Latin times. In Italian, g- was used to represent this, but in other languages j- took the job. This usage is attested earliest in Spanish, where it was in place before 1600.

No word beginning with J is of Old English derivation. [OED]

English dictionaries did not distinguish words beginning in -i- and -j- until 19c., and -j- formerly was skipped when letters were used to express serial order.

In Latin texts printed in modern times, -j- often is used to represent Latin -i- before -a-, -e-, -o-, -u- in the same syllable, which in Latin was sounded as the consonant in Modern English you, yam, etc., but the custom has been controversial among Latinists:

The character J, j, which represents the letter sound in some school-books, is an invention of the seventeenth century, and is not found in MSS., nor in the best texts of the Latin authors. [Lewis]

In English words from Hebrew, -j- represents yodh, which was equivalent to English consonantal y (hence hallelujah) but many of the Hebrew names later were conformed in sound to the modern -j- (compare Jesus).

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K 

eleventh Roman letter, from Greek kappa, from Phoenician kaph or a similar Semitic source, said to mean literally "hollow of the hand" and to be so called for its shape.

Little used in classical Latin, which at an early age conformed most of its words (the exceptions had ritual importance) to a spelling using -c- (a character derived from Greek gamma). In Late Latin, pronunciation of -c- shifted (in the direction of "s"). Greek names brought into Latin also were regularized with a -c- spelling, and then underwent the Late Latin sound-shift; hence the modern pronunciation of Cyrus, Circe. To keep their pronunciation clear, the many Greek words (often Church words) that entered Latin after this shift tended to take Latin -k- for Greek kappa.

K- thus became a supplementary letter to -c- in Medieval Latin, used with Greek and foreign words. But most of the languages descended from Latin had little need of it, having evolved other solutions to the sound shifts.

K- also was scarce in Old English. After the Norman conquest, new scribal habits restricted -c- and expanded the use of -k-, which began to be common in English spelling from 13c. This probably was done because the sound value of -c- was evolving in French and the other letter was available to clearly mark the "k" sound for scribes working in English. For more, see C.

In words transliterated from Arabic, Hebrew, Turkish, Japanese, Hawaiian, etc., it represents several different sounds lumped. In modern use some of them are now with kh-; in older borrowings they often followed traditional English spelling and were written with a C- (Corea, Caaba, etc.).

As a symbol for potassium, it represents Latin kalium "potash." In CMYK as a color system for commercial printing it means "black" but seems to stand for key in a specialized printing sense. Slang meaning "one thousand dollars" is 1970s, from kilo-. K as a measure of capacity (especially in computer memory) meaning "one thousand" also is an abbreviation of kilo-.

As an indication of "strikeout" in baseball score-keeping it dates from 1874 and is said to represent the last letter of struck. The invention of the scorecard symbols is attributed to English-born U.S. newspaperman Henry Chadwick (1824-1908) principally of the old New York "Clipper," who had been writing baseball since 1858, and who explained it thus:

Smith was the first striker, and went out on three strikes, which is recorded by the figure "1" for the first out, and the letter K to indicate how put out, K being the last letter of the word "struck." The letter K is used in this instance as being easier to remember in connection with the word struck than S, the first letter, would be. [Henry Chadwick, "Chadwick's Base Ball Manual," London, 1874]
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