This category lists operating systems (OSs) based on open source code. Some such OSs have closed source variants (BeOS, DOS, Unix, VMS), or straddle these two licensing models (QNX). Open Source computer programs (OSs are programs) are those that include source code, and/or for which the source code is freely available, or made available without a fee or limited permissions. On this page, OSs are arranged in three groups and levels: 1) Top group: types or classes of OS. 2) Middle group: OSs for which there are more than one instance of an OS of this name/type, an OS family. 3) Bottom group: specific OSs, individual instances; there is only one OS of this name/type. These links occupying the top of this page, above the links with descriptions, lead to other directory categories which hold open source OSs which have other, more distinctive traits that identify them as members of some class of OS found in the current directory taxonomy. OSs listed in this category are those that have no other distinctive traits by which to classify them in the current taxonomy. These all seem to have monolithic kernels. An OS is listed in this category if there seems no clearer, more specific category for it. This category also has some other links that are hard to fit elsewhere, of which there are always a few.