wn ▁for ▁the ▁initi ative . ▁The ▁comp lications ▁are ▁such ▁that ▁David ▁Bron stein ▁suggested ▁that ▁the ▁term ▁" def ense " ▁does ▁not ▁fit , ▁and ▁that ▁the ▁name ▁" Ch ig or in ▁C ounter att ack " ▁would ▁be ▁more ▁appropriate . ▁The ▁Two ▁Kn ights ▁has ▁been ▁adopted ▁as ▁Black ▁by ▁many ▁ag gress ive ▁players ▁including ▁Mik h ail ▁Ch ig or in ▁and ▁Paul ▁K eres , ▁and ▁world ▁champions ▁Mik h ail ▁Tal ▁and ▁Bor is ▁Sp ass ky . ▁In ▁modern ▁grand master ▁play , ▁ 3 . B c 4 ▁is ▁far ▁less ▁common ▁than ▁ 3 . B b 5 , ▁and ▁the ▁more ▁solid ▁ 3 ... B c 5 ▁is ▁the ▁usual ▁reply , ▁so ▁the ▁Two ▁Kn ights ▁Def ense ▁is ▁inf re qu ently ▁seen . ▁It ▁remains ▁popular ▁with ▁amateur ▁players . ▁The ▁theory ▁of ▁this ▁opening ▁has ▁been ▁expl ored ▁extens ively ▁in ▁correspond ence ▁ch ess ▁by ▁players ▁such ▁as ▁Hans ▁Berliner ▁and ▁Y akov ▁E str in . ▁ ▁Main ▁variations ▁▁ 4 . N g 5 ▁ ▁German ▁master ▁Sieg bert ▁T arr as ch ▁called ▁ 4 . N g 5 ▁a ▁" real ▁du f fer ' s ▁move " ▁( ein ▁richt iger ▁St ü m per zug ) ▁and ▁Soviet ▁opening ▁the or ist ▁Vas ily ▁Pan ov ▁called ▁it ▁" prim itive ", ▁but ▁this ▁attack ▁on ▁f 7 ▁pract ically ▁wins ▁a ▁pa wn ▁by ▁force .