A text file does not look like much of an invention. Open one and there are no page margins, embedded photographs, tracked changes, or font choices. There are only characters and the boundaries between lines. That sparseness is the reason the format has remained useful through several generations of hardware.
Long before the .txt suffix became familiar on home computers, machines were already exchanging character data. Teleprinters and terminals encouraged a view of text as a stream of codes. ASCII, standardized in the 1960s and revised over the following years, gave many systems a common representation for English letters, digits, punctuation, and control characters. Its reach was limited, but its influence was enormous. The first 128 characters of Unicode still correspond to ASCII.
The extension is only a label
DOS and Windows made filename extensions particularly visible, and .TXT became shorthand for a file that a basic editor could open. Other operating systems have also used file metadata, content inspection, or naming conventions without extensions. In every case, the suffix itself does not transform the contents. Renaming a word-processing document from .docx to .txt leaves the original document structure in place; it merely gives the file a misleading name.
Nor is “plain text” a complete technical specification. Software still needs to know how bytes map to characters. A file written using one regional encoding may be unreadable when opened as another. Systems have historically disagreed about how to mark the end of a line. Tabs do not have a universal display width. These details account for a surprising number of damaged accents, stray symbols, and oddly broken lines.
Unicode made plain text broader
Unicode addressed the impractical idea that every language should live in a separate character set. UTF-8, one way of encoding Unicode characters as bytes, proved especially useful because ordinary ASCII text is already valid UTF-8. It can represent scripts from around the world while remaining compatible with a great deal of older software and data.
That does not mean every program handles every character equally well. The chosen font may lack a glyph, text direction can affect display, and normalization can give visually identical strings different underlying sequences. Plain text is simple compared with a formatted document, not free of technical decisions.
Why people keep using it
A text file can be searched, compared, copied, compressed, emailed, processed by a script, or placed under version control. If one editor disappears, many others can take its place. The format is well suited to notes, source code, logs, configuration, and material intended to survive a change of software.
The trade-off is equally plain. TXT does not preserve a heading as a heading or remember that a sentence was set in italics. Writers often devise conventions—capital letters, blank lines, asterisks—to suggest structure, but those are characters rather than formatting. When layout is essential, another format is more appropriate. When continued access to the words is the priority, plain text remains difficult to improve upon.
Technical reference: The Internet Engineering Task Force describes the text/plain media type in RFC 2046.