I think you mean the LGPL? It allows you to "convey a combined work under terms of your choice" as long as the LGPL-covered part can be modified, which can be a

I think you mean the LGPL? It allows you to "convey a combined work under terms of your choice" as long as the LGPL-covered part can be modified, which can be achieved either via dynamic linking or by providing the proprietary code as bare object files to relink statically. The GPL doesn't have this exception.