Who invented file extensions in file names?

Question

Do we know when and where the idea of adding a suffix to filenames was conceived? I have found a lot of information about the history of specific file formats, but I am curious about when the need for separating files by type arose and who got the idea to put that information in the names of the files.

These might be separate questions, but I would also want to know when the convention to add the file extension as a dot followed by a suffix was created, and, if that is possible to know, what the first file extensions were.

Answer 1

The MIT Compatible Time-Sharing System (CTSS), demonstrated in 1961, had two "names" for each file, with the second telling CTSS how to process the file. This had the function of a filename extension, but was apparently not written with a dot separator.

Examples included "MAD" for source code for the Michigan Algorithm Decoder programming language, "FAP" for the FORTRAN assembly program, "SYMTB" for symbol tables and, apparently, "BSS" for the .bss segment.

It appears that CTSS was able to run programs for the IBM FORTRAN Monitor System, which served as a basis for parts of CTSS.

IBM's CMS (originally Cambridge Monitor System, now Conversational Monitor System, part of z/VM), was influenced by CTSS and still uses two-part filenames, called "filename" and "filetype". The MVS and z/OS line of IBM mainframe operating systems uses a dot as a directory separator, and doesn't use filetypes. The DOS/VSE line doesn't use filetypes either.

The MS-DOS style of filename and extension descends (via CP/M) from DEC operating systems, which took inspiration from CTSS, as Walter's answer explains.

Some systems allowed filenames on tapes, but filenames became much more important once disk drives were in general use. CTSS was first demonstrated in November 1961, plausibly with the IBM 1301 disk drive which had been announced in June and held 28 million (6-bit) characters. That was the first disk with a head per surface, which reduced maximum access time to 180 milliseconds, making a dir command actually useful.

Answer 2

The PDP-6 multiprogramming monitor, an early operating system first delivered in 1964, used the term "filename extension" for the second name attached to a file, usually indicating the type. It also used the dot character as preceding the extension.

The linked document explains the monitor, although not in detail.
Multiprogrammed monitor

In that document, under FILE DELIMITERS, you'll see that dot precedes a filename extension.

The people who built the monitor were undoubtedly influenced by the CTSS system at MIT. Some of the early PDP-6 team were veterans of the Tech Model Railroad Club, and had persuaded the designers to add four stack oriented instructions to the repertoire.

Edit: The PDP-10 was a follow-on processor to the PDP-6, and the monitor added support for it. Eventually, the monitor was renamed to TOPS-10. The dot convention spread to the other operating systems within DEC. CP/M and MS-DOS also used the dot to separate the two parts of a filename.

Bill Gates may have first been exposed to the filename.ext convention when he worked on a PDP-8 in prep school.

Answer 3

In addition to Walter's spot on answer about the combined use of extensions and delimiting them by a dot and John's about the CTSS (*1) as origin for user defined types, it might be worth to note that the concept grew out of more humble beginnings. It also was (and in some cases still is) different from what the question assumes.

[Who invented] file extensions in file names?

Historical file name extensions are not in a file name - considering 'in' as part of the file name - but a separate attribute to a file - much like date created or owner.

Do we know when and where the idea of adding a suffix to filenames was conceived?

Please see Walter's answer for the direct ancestor of the modern file type and handling.

I am curious about when the need for separating files by type arose

The beginning may even predate nearly all of what we would nowadays call OSes, and originates with the first usable machine-controlled (*2) mass storage, aka Tape Drives. Here headers helped to describe the way records are accessed within a file, most notably fixed length vs. variable length vs block access.

That way, a user program could read records from tape the same way independent of format (*3). With disks more complex types were introduced, like QSAM, ISAM, VSAM, etc., those types were coded in dedicated fields and accessed by commands or command parameters separate from the file name.

Most notable here is that this type scheme grew bottom up, out of necessity to support certain FS services - like the mentioned record access - and strictly limited to what the OS/FS could understand.

In contrast, the second file name of the CTSS was developed in a top down fashion to provide a general purpose field, not intended for (primary) OS interpretation for structure but marking purpose for applications to handle them.

Follow-up systems to that went two ways, either going

• the DEC way (*4) of using two dedicated fields of Name and Type with the latter being handled as part of the file name delimited by a dot. Or they followed

• the UNIX way (*4) of dropping that extension field, handling only a single name field with no further structure.

The DEC way led to CP/M and in turn MS-DOS. Here all user interaction was shaped to make the dedicated field look as if it was part of the name field, thus the general impression of an extension being part of the name. While those later extending the Name part to 8 chars (DEC was originally 6), the extension was kept as 3 characters, creating the well known 8+3 structure excluding the dot.

The dot was never stored on media or in (FS) data structures. It's only a delimiter for input parsing and added on output for symmetry/convenience. Likewise the name part is always 8 characters and filled up with spaces.

An interesting variant of the DEC way is the Macintosh MFS and follow up HFS as it manages not one but two file 'types', four bytes each. One noting the creator and one for the type, but none of them directly visible to the user.

who got the idea to put that information in the names of the files.

Strictly that only happened with systems based on a flat file concept - most notable the UNIX-FS - that is, systems that do not support different file types as part of the file system. In those systems a File is just a blob of data. Giving it any descriptive meaning is up to applications (*5). Any file extension used is not a dedicated file type in its own right, but a naming convention to structure a single file name including the dot.

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Bottom line, even a simple looking question like this can have a very complex background :))

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*1 - Both are worth an upvote.

*2 - "Machine-controlled" meaning read and/or written with full positioning control (rewind/reposition) on the same media.

*3 - Much as were stacks of punched cards. In fact, exactly the same way as device assignment to program input has always been an operator/OS task. Most (not too complex) mainframe programs do not handle filenames at all but read from and write to standard handles. A bit like with Unix, except that there is more than one standard handle available for each.

*4 - Names used as generic indicators of heritage, not necessary inventor.

*5 - Or other dedicated fields, like the executable flag in unixoid FSes.

Answer 4

The PROMPT (disc)/POST (magtape) file maintenance systems, on English Electric KDF9 in the mid-1960s, used a 12-character alphanumeric file identifier. Not up to 12 chars, but 12 chars whether you wanted them or not. The same file system layout was adopted for Leeds University's Eldon2 multiaccess system.

Of the 12 characters, the last 3 conventionally encoded the 'type' of a file.

1-char language identifier: A for Algol, U for Usercode (assembler), and that's all the choices you needed

1-char type of code: P for Program, S for Segment

1 char for number of store modules (i.e., size), U for Universal (i.e., not applicable)

So Algol program names typically ended with APU.

No dot involved though.

Answer 5

The filestore of GEORGE 3 (ICL 1900 systems) had a four-character language code for each file.

This was not considered to be part of the file name, but was part of an 'entrant description' (file specification in other systems) when a file must be identified to the system.

For example, a Fortran prog might be :DAVE.MYPROG(/FORT), where :DAVE is the user identifier, MYPROG is the file name, and FORT says it's Fortran source.

As far as I recall, GEORGE didn't care whether you used language types or not, but there were assumptions built into some programs, much like a typical compiler front-end today assumes that .c means C source.

Note that there's a dot in the entrant description, but it's not before the file 'extension'.

While this is deviating from being a strict answer to what I think was the actual question asked, it demonstrates that the problem solved by "dot extension" was one that every file system tended to recognize as important.

GEORGE 3 was implemented from about 1965 onwards; the tree-structured file system was inspired by the well-known 1965 paper by Daley and Neumann.

Answer 6

The IBM 1130 had five character file names (this is how FORTH got its name). Although the official character encoding was 8 bit EBCDIC, names were encoded as pairs of 16 bit words using 6 bit characters. This left two extra bits, used in file names to encode the type of the file. There were three types: object files, executable files, and data files. Source was stored as decks of cards, not files.

Answer 7

A supplement for answers of Walter Mitty and Raffzahn. Multics filenames also use suffixes (or called 'last component') started with a dot, such as .fortran, .pl1 and .runoff.

I don't know whether these suffixes are same to the concept of CMS filetype or DEC/MS-DOS file extensions. If these suffixes are also file extensions, I don't know which of DEC way and Multics influenced another, or which exists earlier, or they have separated or same sources, because the beginning time of the development of Multics is close to the year PDP-6 Monitor is released.

Multics Commands, Page 3-688, the 'suffix' command.

Answer 8

The first known use of filename extensions was in the PDP-6 multiprocessing monitor, an early operating system first delivered in 1964. The PDP-6 monitor used the dot character as preceding the extension, and the convention later spread to other operating systems, such as CP/M and MS-DOS.

Bill Gates, the co-founder of Microsoft, has said that he was first exposed to the filename extension convention while using the PDP-10 operating system at Harvard University in the early 1970s.