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Diamond alive in the corner
Diamond dead in the corner
Diamond in the corner
Double Ko kills diamond in the corner
[Kajiwara Takeo] - [Fujisawa Hideyuki], 1958-05-21
iopq: A diamond survives in the corner. It's sometimes useful for invasions or ko threats to know this. Don't let one complete in the corner! Circled points are ko threats for white.
White can move in the marked point to kill Black, just knowing about the diamond in the corner! If the diamond does not complete, Black dies.
Bill: I believe that this is the diamond shape. (See T. Wolf, The diamond. British Go J. 108 (1997) 34-36.)
In contrast, a ponnuki is the process of capturing a single stone leaving such a shape, not the shape itself. 
Difference between Ponnuki and Diamond
Charles Matthews: Ponnuki implies local tally BBB (four black stones played, one white captured, 4 - 1 = 3). If you make a diamond as Black without capturing a white stone that is local tally BBBB. The former is proverbially worth 30 points, i.e. ten points when you divide by the tally; the latter would therefore give only 7.5 points per play.