diff --git a/README.md b/README.md
index c1229844..1dbf3150 100644
--- a/README.md
+++ b/README.md
@@ -452,6 +452,108 @@ sed 's/a/x/g' > /dev/null  0,00s user 0,00s system 43% cpu 0,029 total
 rr x y myfile -o > /dev/null  0,21s user 0,04s system 86% cpu 0,294 total
 ```
 
+## Hairy details
+
+Each part of a run is a stack of layers. Each layer is optional, and works on what the layer before gives it. So the flags work together by their order, not by special rules.
+
+### On the way in
+
+#### The pattern
+
+1. The pattern as typed. `€` is `$` and `§` is `\`.
+2. `--literal` (`-L`) makes it plain text, so `.` is a dot.
+3. `--void-ignore-case` (`-I`), `--void-multiline` (`-M`), `--dot-all` (`-s`), `--unicode` (`-u`) and `--void-global` (`-G`) decide how the regex works.
+
+```bash
+> echo -n 'a.b' | rexreplace '.' '_'
+___
+> echo -n 'a.b' | rexreplace '.' '_' --literal
+a_b
+```
+
+#### The replacement
+
+1. The replacement as typed, with `€` and `§` as above. Or the piped data with `--replacement-pipe` (`-R`), used as it is.
+2. `--js` (`-j`) runs it as code, and the value of the code goes on. `--js-pipe` also gives the code the piped data as `pipe`.
+3. `--replacement-file`: what came so far is a path, and the content of that file goes on.
+4. `--trim-file` trims it.
+5. It goes into the text. `$1` is group 1, `$&` the whole match, and `$$` one `$`, unless the value came from `--js` code, or `--literal-replacement` (`-l`) is given.
+
+```bash
+> echo -n foo | rexreplace '(f)(oo)' '$2$1'
+oof
+> echo -n foo | rexreplace 'o' '$0.toUpperCase()' --js
+fOO
+> echo -n PRICE | rexreplace '(PRICE)' 'only $1.99'
+only PRICE.99
+> echo -n PRICE | rexreplace '(PRICE)' 'only $1.99' --literal-replacement
+only $1.99
+> echo -n 'a {{footer}} b' | rexreplace '\{\{footer\}\}' partials/footer.html --replacement-file --trim-file
+a <footer> b
+```
+
+As the layers are in order, they can be put together. Here the piped data is code that gives the path of the file to use:
+
+```bash
+> echo '"partials/footer.html"' | rexreplace '\{\{footer\}\}' _ page.html -R --js --replacement-file --trim-file
+```
+
+Code that lives in a file is loaded with `r()` in `--js`, like `'r("fix")($0)'`.
+
+#### The files
+
+1. The paths and globs given as arguments, and the lines piped in with `--glob-pipe` (`-g`). `--null-in` splits the piped lines on null bytes.
+2. A path that is a file is used as it is. The rest are globs.
+3. The globs ignore case, unless `--void-ignore-case` (`-I`).
+4. `--exclude-glob` (`-X`) takes away the files its globs find.
+5. `--full-path` (`-f`) makes each path full.
+6. `--exclude-re` (`-x`) takes away the files with a path that matches.
+7. `--sort` decides the order, and `--sort-reverse` turns it around.
+
+```bash
+> rexreplace foo bar 'docs/**/*.md' --exclude-glob 'docs/old/**' --exclude-re draft --sort size
+```
+
+### On the way out
+
+1. What is printed, and where:
+
+| Flags | stdout | stderr |
+| --- | --- | --- |
+| (none) | the names of the changed files | |
+| `--output` (`-o`) | the content | the names |
+| `--output-match` (`-m`) | the matches | the names |
+| `--rename --simulate` | the new names | the old names |
+
+```bash
+> rexreplace o x a.file b.file -o 2>/dev/null
+fxx
+fxx
+> rexreplace o x a.file b.file -o 2>&1 >/dev/null
+a.file
+b.file
+```
+
+2. Between the items: `--name-delimiter` goes after each name, and `--content-delimiter` after each content or match. `--delimiter` sets both, and `--null-out` is `--delimiter` with a null byte. The most specific one wins. Without a delimiter it is a line break, unless the item already ends with one.
+3. The tails: the delimiter also goes after the last item, unless `--no-name-delimiter-tail`, `--no-content-delimiter-tail` or `--no-tail`. Piped data that is printed is left as it came in, with nothing after it.
+
+```bash
+> rexreplace o x a.file b.file --name-delimiter ', ' --no-name-delimiter-tail
+a.file, b.file
+> rexreplace o x a.file b.file --null-out | xargs -0 ls -l
+```
+
+4. `--nl` ends stdout and stderr with a line break, if they do not end with one.
+5. How much: `--quiet` (`-q`) shows only the errors (❌), and no names or warnings (🟡). `--quiet-total` (`-Q`) shows nothing, and `--verbose` (`-V`) tells each step to stderr. An error that stops the run ends with `No further processing.`
+6. The status report: `--status` and `--status-compact` (`-c`) print it as the last thing, also with `--quiet-total`. `--debug` (`-d`) prints only the status report, with what went into the engine and what it did.
+7. The exit code, see [Exit codes](#exit-codes).
+
+### Other details
+
+- `--rename` (`-r`) works on the name of each file, and `--rename-path` on the whole path, so a `/` moves the file. If planned renames clash the process is halted and no change is applied to any file.
+- `--output-match` only prints what the pattern finds. The replacement is not used, so `--js` code is not run, a replacement file is not read, and `--rename` renames nothing.
+- The files are handled at the same time, so names can come in another order than the sort. They are handled one at a time, in order, with `--output`, `--output-match`, `--void-async` (`-A`), `--sort`, or when `--js` code uses `$i`, `$c`, `$m` or `$M`.
+
 ## Test
 
 ### Regression
diff --git a/devguide.md b/devguide.md
index dc72729c..b27fc433 100644
--- a/devguide.md
+++ b/devguide.md
@@ -33,6 +33,14 @@ All output goes through `src/output.ts`. Pick the function by who the message is
 - `warn()`, `error()`, `checkFailed()` and `panic()` add to the status report as they go, also when the quiet flags hide what they print.
 - The exit code is 0 when the run is done, 1 when a check from `--expect-*` is not met, 2 when a file gave an error, and 3 for a panic or wrong flags. See Exit codes in the README.
 
+## Layers
+
+Each part of a run is a stack of layers: the pattern, the replacement and the files on the way in, and what is printed on the way out. They are told in "Hairy details" in the README. Use them as a guiding principle:
+
+- To understand what happens in a run.
+- To write tests: one for each layer, and for the ways they combine.
+- To challenge a new flag: it should be a layer, or change one layer, and not need its own rules for how it works with the other flags.
+
 ## Tests
 
 - **Tests are strict, lean and focused too.** One thing per test, and the name of the test says what it is.