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The rules of composing valid file names (7.9.3).

A valid file specification consists of an optional drive letter (which is
always followed by a colon), a series of optional directory names separated
by backslashes, and a file name.

FAT File System:  Directory names and file names can contain up to eight
characters followed optionally by a period and a three letter extension.
 The complete path (including drive, directories and file name) cannot
exceed 143 characters.  Case is ignored (lowercase letters are converted to
uppercase letters).

HPFS File System:  Directory names and file names can contain up to 254
characters in the OS/2 High Performance File System (HPFS).  However, the
complete path (including drive, directories and file name) cannot exceed 259
characters.  The period is a valid file name character and can appear in a
file name or directory name as many times as required; HPFS file names do
not require file extensions as in the FAT file system.  The HPFS preserves
case in file names only in directory listings but ignores case in file
searches and other system operations (i.e, a directory cannot have more than
one file whose names differ only in case).