diff --git a/doc/stdenv/cross-compilation.chapter.md b/doc/stdenv/cross-compilation.chapter.md
index 7b8f2b4ce6cdb..3cef3a92ed51d 100644
--- a/doc/stdenv/cross-compilation.chapter.md
+++ b/doc/stdenv/cross-compilation.chapter.md
@@ -44,7 +44,29 @@ The exact schema these fields follow is a bit ill-defined due to a long and conv
 
 `config`
 
-: This is a 3- or 4- component shorthand for the platform. Examples of this would be `x86_64-unknown-linux-gnu` and `aarch64-apple-darwin14`. This is a standard format called the "LLVM target triple", as they are pioneered by LLVM. In the 4-part form, this corresponds to `[cpu]-[vendor]-[os]-[abi]`. This format is strictly more informative than the "Nix host double", as the previous format could analogously be termed. This needs a better name than `config`!
+: This is a 3-, 4-, or 5-component shorthand for the platform.
+
+  Format:
+
+  - `[cpu]-[vendor]-[os]`
+  - `[cpu]-[vendor]-[os]-[abi]`
+  - `[cpu]-[vendor]-[os]-[libc][abi]`
+
+  :::{.note}
+  There is no hyphen separating the `[libc]` field from the `[abi]` field.
+  :::
+
+  Examples:
+
+  - `aarch64-apple-darwin14`
+  - `x86_64-unknown-linux-gnux32`
+  - `mips64el-unknown-linux-muslabin32`
+
+  This is a standard format [pioneered by autoconf](https://www.gnu.org/software/autoconf/manual/autoconf-2.65/html_node/System-Type.html#System-Type) and [adopted by LLVM](https://clang.llvm.org/docs/CrossCompilation.html#target-triple).
+  It is strictly more informative than the platform string `[cpu]-[os]` used by Nix.
+  <!-- TODO: link to a specification of the platform string used by Nix in the Nix manual, once such a thing exists -->
+
+This field should be *canonicalized*.  The rules for canonicalizing a tuple are kept in the `config.sub` file in the source code for `gnu-config`.
 
 `parsed`