> Source: https://txtfetch.com/glossary/byte-order-mark
> Plain-text twin — every page on txtfetch.com has one. https://txtfetch.com/text

---

# Byte order mark (BOM)

A few invisible bytes at the start of a text file declaring its encoding and byte order. Helpful for a reader, a stray character if a parser doesn't strip it.

definition

A byte order mark (BOM) is an optional sequence of bytes placed at the start of a text file. It declares the file's encoding and, for multi-byte encodings, the byte order used.

Also called: BOM, UTF-8 BOM

in-plain-terms

UTF-16 can store each character's bytes in two different orders. A reader has no way to tell which one a file uses without being told. The BOM's job is to say so before any real content begins. UTF-8 doesn't need a BOM for this reason, since it has no byte-order ambiguity. But some tools write one anyway, mostly for legacy compatibility.

A reader that doesn't recognize and strip the BOM treats it as ordinary content. It shows up as an invisible or oddly-rendered character glued to the very first word of the file. This is a common, easy-to-miss extraction artifact.

why-it-matters

-   A stray BOM at the start of extracted text can silently break an exact-match comparison or a first-line parser. It can also break a chunk boundary that assumes the file starts cleanly.
-   Detecting and stripping the BOM correctly is also how a reader knows which encoding and byte order to apply to the rest of the file. Get it wrong and every character after it can decode incorrectly too.

how-to-check

-   Check the first few bytes of a text file for a UTF-8 or UTF-16 BOM signature before assuming the file starts with real content.

related-terms

-   [Character encoding →](https://txtfetch.com/glossary/character-encoding)
-   [Mojibake →](https://txtfetch.com/glossary/mojibake)

faq

**Does every UTF-8 file have a BOM?**: No. A BOM is optional in UTF-8 and isn't needed for it, since UTF-8 has no byte-order ambiguity to resolve. Some tools write one anyway for legacy compatibility, which is why a reader still needs to detect and strip it.

**What happens if a BOM isn't stripped?**: It gets treated as ordinary content. It typically shows up as an invisible or oddly-rendered character glued to the very first word of the extracted text. That can break an exact-match comparison or a first-line parser downstream.

related-reading

-   [Extract text from CSV →](https://txtfetch.com/extract/csv)
-   [Fix: mojibake and wrong-encoding text →](https://txtfetch.com/fixes/mojibake-wrong-encoding)
-   [All glossary terms →](https://txtfetch.com/glossary)

Characters & encoding

-   [Character encoding →](https://txtfetch.com/glossary/character-encoding)
-   [Mojibake →](https://txtfetch.com/glossary/mojibake)
-   [Ligature →](https://txtfetch.com/glossary/ligature)
-   [Replacement character →](https://txtfetch.com/glossary/replacement-character)
-   [All terms →](https://txtfetch.com/glossary)

## See the term in real output.

Drop a file into the free reader and watch it happen.

[Open the file reader →](https://txtfetch.com/tools/file-to-text)

[Get an API key →](https://app.txtfetch.com/signup)