Two black Bi-valves, involving the same two lines, in such way that the line, which is opened in the first line of play, is closed in the second and vice versa.
See also: Bi-valve.
Alias: Reciprocal bi-valve.
Belgrade Internet Tourney, 2012
b) 1.Sg3+ Kxf4 2.Re7 Qa8#
1.Sd7 Re4+ 2.Kxf5 Ra5#