Chains
Costs7 Wonders Duel
Some Buildings grant a white chain symbol. Some Age II and III Buildings show a matching white symbol in their cost area, under any resource cost. If your city already contains the Building that carries that symbol, you can construct the new Building for free.
Ask RuleRunner about 7 Wonders Duel
Also answers
chain symbollinked buildingslink symbolfree constructionfree construction conditionhow do i build for freehow do i get buildings freewhite symbolwhite symbol on cardmatching chain symbol
More Costs rules
All 7 Wonders Duel rules and how to play · More about 7 Wonders Duel