Here I report on code quality metrics I collected from four large industrial-scale operating systems: FreeBSD, Linux, OpenSolaris, and the Windows Research Kernel (WRK). The main contribution of this research is the finding that there are no significant across-the-board code quality differences between four large working systems, which have been developed using various open-source and proprietary processes. An additional contribution involves the proposal of numerous code quality metrics for objectively evaluating software written in C. Although these metrics have not been empirically validated, they are based on generally accepted coding guidelines, and therefore represent the rough consensus of developers concerning desirable code attributes.