Introduced with OS/2 2.0, these can be identified by the "LX" in ASCII. These can only be run by OS/2 2.0 and higher. They are also used by some DOS extenders.
Mixed 16/32-bit Linear Executable
Introduced with OS/2 2.0, these can be identified by the "LE" in ASCII. This format is not used for OS/2 applications anymore, but instead for VxD drivers under Windows 3.x and Windows 9x, and by some DOS extenders.
Introduced with Windows NT, these are the most complex and can be identified by the "PE" in ASCII (although not at the beginning; these files also begin with "MZ"). These can be run by all versions of Windows and DOS (DOS runs the MZ section, Windows runs the NE or PE section). Using HX DOS Extender DOS can load the NE and PE sections. They are also used in BeOS R3, although the format used by BeOS somewhat violates the PE specification as it doesn't specify a correct subsystem. These can also be used on ReactOS
64-bit Portable Executable (PE32+)
Introduced by 64-bit versions of Windows, this is a PE file with wider fields. In most cases, code can be written to simply work as either a 32 or 64-bit PE file. 
Besides these, there are also many custom EXE formats, such as W3 (a collection of LE files, only used in WIN386.EXE), W4 (a compressed collection of LE files, only used in VMM32.VXD), DL, MP, P2, P3 (last three used by Phar Lap extenders), and probably more.
When a 16-bit or 32-bit Windows executable is run by Windows, execution starts at either the NE or the PE, and ignores the MZ code. On the other hand, DOS cannot execute these files (except using HX DOS Extender, which supports PE files only). To prevent DOS from crashing, all Windows executable files should and usually do start with a "working" DOS program called a stub.,[self-published source?] simply displaying the message "This program cannot be run in DOS mode" (or similar) before exiting cleanly. A few dual-mode programs (MZ-NE or MZ-PE) (such as regedit and some older WinZIP self extractors) include a more functional DOS section.