background beatbackground carboxyhemoglobin levelbackground checkbackground concentrationbackground conditionsbackground countbackground graphicsbackground in communicationbackground informationbackground information about anybodybackground knowledgebackground levelbackground lightbackground materialbackground musicbackground noisebackground paperbackground polarizationbackground radiationbackground seismicitybackground subtractionbackground temperaturebackground tintbackground variablesbackgroundedbackgrounderbackgroundingbackgroundsbackhandbackhand castbackhand weldingbackhandedbackhanded complimentbackhanderbackhaulbackhauledbackhaulingbackhoebackingbacking and fillingbacking awaybacking beadbacking downbacking firebacking gasbacking offbacking passbacking powderbacking ringbacking shoe