The SiO2'–CaO/(CaO + K2O) (S'CK) diagram is an empirically derived major element-based equivalent to the modal IUGS alkali feldspar–quartz–plagioclase classification scheme for granitoids. It employs the content of SiO2 and CaO/(CaO + K2O) ratio to approximate the IUGS classification diagram and a normative-based Q'–ANOR plot. Four trends have been superimposed onto the SiO2'–CaO/(CaO + K2O) diagram based on published datasets from the Peninsular Ranges (calcic: C), Tuolumne (calc–alkalic: CA), Sherman (alkali–calcic: AC), and Bjerkreim-Sokndal (alkalic: A) batholiths, which were employed to constrain the positions of the C–CA, CA–AC and AC–A suite boundaries on the SiO2 versus (Na2O + K2O – CaO) (or modified alkali–lime index, MALI) granitic classification diagram. A merit of the SiO2'–CaO/(CaO + K2O) plot is identifying rock types comprising a suite and their relative abundances. The distinguished projections of five typical granitoid assemblages, which are summarized by Bonin et al. (2020), demonstrate the ability of SiO2'–CaO/(CaO + K2O) diagram to decipher their petrogenesis. The SiO2' –CaO/(CaO + K2O) plots for the plutonic suites of 'known' tectonic settings can reveal their evolution paths and the lithological statistics. Accordingly, it is suggested that the SiO2'–CaO/(CaO + K2O) plot can distinguish the tectonic environments of plutonic suits by comparing the plutonic suites or batholiths of 'unknown' tectonic context to the published datasets from granitoid suites formed within 'known' tectonic settings. The modified SiO2'–CaO/(CaO + K2O) diagram links the bulk chemical composition of granitoid suites to the likely source, magmatic evolution, and tectonic setting; thus, it may be a useful tectono-magmatic classification scheme for granitoid suites.