I CAN HAZ WORD TRIANGLE?
A 7-word triangle from The Enigma (KU-1 in October 2009)
Noam D. Elkies, 27 August 2009

SOLUTION

1 2 3 4 5 6 7
K    1   Exactly 1/1048576 of a gig
F T    2   About 1/3280.84 of a km
H R H    3   Mini style for the Prince of Wales
X B O X    4   Halo platform
H B O M B    5   Bikini première
F R O M M Y    6   Dreams ______ Father
K T H X B Y E    7   Lolcat’s leave-taking

No separate Down clues: this triangle symmetrical about a NW-SE diagonal axis, and the same entries appear Across and Down.

THX to Scott Kominers for telling me of the 7-letter “word” and daring me to put it in such a puzzle. Before constructing it, I came up with an upside-down 7-triangle where KTHXBYE was the first word clued rather than the last: 
  KTHXBYE
  TIERCE
  HELAS
  XRAY
  BCS
  YE
  E
The setting with XBOX (rather than XRAY) appeared in the October 2009 issue of The Enigma, but with a KU caveat (KU = “Kreweland Unusual”), offered for possible enjoyment and kudos but not counted towards the issue’s solving statistics, because it features a “word” that cannot be found in standard reference sources.
The Enigma is the word-puzzle monthly published by the National Puzzlers' League. Each month’s offerings include a few mini-crosswords called “forms”, which are generalizations of word squares. The conventions for forms differ somewhat from what crossword regulars may be familiar with: each letter is crossed, but forms may include 2- and even 1-letter words (as seen in this triangle), and forms with a NW-SE diagonal axis of symmetry usually have the same words Across and Down; and solvers are expected to use references, so obscure words and names can be tolerated and even welcomed, but must be findable in standard dictionaries and similar references (such as the Grove Dictionary for musical items), else the puzzle will likely get KU’d. [The NPL also uses its own jargon for some of the form shapes, which does have the advantage of distinguishing among the four possible kinds of triangle: the shape of this “unusual but not cruel” triangle is called a “right halfsquare”, and the XRAY one would be an “inverted left halfsquare”. The other two kinds are usually equivalent, but occasionally there’s a reason to prefer one of the two possible orientations.]
1A – gig = gigabyte = 220 Kilobytes = exactly 1048576 K (also KB, but never Kb = Kilobit, nor kB because lowercase kilobyte is not 210=1024 but 103=1000 bytes — see Wikipedia’s entry on the kilo- prefix). By crossword convention, the abbreviation in the clue suggests an abbreviated answer; likewise for the second entry (the third clue indicates the abbreviation directly with “mini”).
2A – There are 1/0.0003048 = 3280.8398950... feet in a kilometer (and yes, I shouldn’t have used “km” in this clue, even if it’s not quite the same kind of “kilo-”…).
3A – The Prince of Wales, heir to the British Throne, is styled “His Royal Highness”, or HRH for short.
4A – XBOX is one of the “Kreweland Unusual” entries in this puzzle, though it could have been justified by citing a New York Times article that ran in the 16 August 2009 issue of the Sunday Magazine.
5A – The first hydrogen bomb was exploded at Bikini Atoll.
6A – Obama’s autobiographical memoir Dreams from My Father is another non-dictionary entry, though it too had a recent New York Times source (Books, 30 August 2009) …
7A – … but urbandictionary.com is beyond the pale. (The K of KTHXBYE is short for OK; THX = thanks; and as usual BYE = good-bye.)