Three or more self-blocks on the same square with the respective level of dual avoidance. Thus we may have triple, quadruple, quintuple avoidance, etc.
See also: Changed blocks.
Bakcsi, György
Probleemblad, 1962
2nd Prize
1.Rf3
Se5
2.Sxe7
Sg4#
1.Sf3
Sd4
2.Sf6
Sf5#
1.f3
Sb4
2.Sh6
Sd5#
Braithwaite, Kenneth Holmes
The Problemist, 1981
Spec. HM
1.Sg6
Se6+
2.Kh5
Sg7#
1.Qg6
Se2+
2.Kh5
Sg3#
1.Rg6
Sg2+
2.Kh5
Rh4#
1.f3 Sb4 (Sd4?, Se5?) 2.Sh6 (Sf6?, Sxe7?) Sd5#
1.Sf3 Sd4 (Se5?, Sb4?) 2.Sf6 (Sxe7?, Sh6?) Sf5#