This category is for operating systems (OSs) with two main traits, they: 1) Have a microkernel architecture. 2) Are open source. Other OSs go elsewhere. On this page, OSs are arranged in two groups and levels: 1) Top group: OSs for which there are more than one instance of an OS of this name/type, an OS family. 2) Bottom group: specific OSs, individual instances; there is only one OS of this name/type.