This wrong: <div class = "person__arm" > <div class = "person__arm--left" > ... </div> <div class = "person__arm--right" > ... </div> </div> There are three elements! Element with modificator should write so: <div class = "person__arm person__arm--left" > ... </div> <div class = "person__arm person__arm--right" > ... </div> And you can use binary notation with modifiers (person__arm_left), or use "type", for example: .person__arm_type_left {} .person__arm_size_big {} .person__arm_theme_main {}