Pygmy hourglass
In a Pygmy hourglass (named for its creator, Pygmy), words read across and diagonally from top left to bottom right. Two diagonally adjacent, equal-length words form the basewords.
To make a grid for a Pygmy hourglass, start with the middle Across entry; that will be two letters long. Then extend the grid in a triangle one square at a time in each direction. (For example, in a Pygmy hourglass with 15 across clues, word 8 is two letters long; words 7 and 9 are three letters; and so on, up to words 1 and 15 at nine letters.)
A triple Pygmy hourglass is the same, except that it has three diagonally adjacent basewords; thus, its central across word is three letters long.