1. CTSS
   1. Multics
      1. UNIX
         1. BSD
            1. 4.4 BSD Lite
            2. FreeBSD
            3. NetBSD
         2. System V
            1. Solaris
2. DOS
   1. PC-DOS
   2. MS-DOS
      1. Windows 16bit
         1. Windows 9x
3. Windows Mobile
   1. Windows Phone
4. QNX
5. Linux
   1. Various Distributions
      1. Debian Family
      2. RedHat Family
      3. ArchLinux
      4. Gentoo
6. Symbian
7. Mach
   1. GNU Hurd
      1. GNU Mach
   2. System x (Mac OS)
8. Android
9. Windows NT
   1. Windows 2000
   2. Windows XP
   4. Windows 10
10. MINIX
    1. MINIX 3
11. iOS
12. Darwin: OS X (macOS)
13. Open in XMind ZEN / XMind 8 to view the important side notes ;)