diff --git a/README.md b/README.md
index e0f9df6..4ba30e7 100644
--- a/README.md
+++ b/README.md
@@ -1,36 +1,147 @@
-## Old Rubies, meet new macOS and Ubuntu
+## Old Rubies, meet new macOS, Ubuntu, and Arch
 
 Trying to run an old, unsupported Ruby? You're in the right place.
 
-This is a collection of ruby-build definitions for local dev on macOS and Ubuntu. Not for production use. macOS (and Homebrew) or Ubuntu are required.
+This is a collection of ruby-build definitions for local dev on macOS, Ubuntu, and Arch
+(including Omarchy). Not for production use.
+
+Available definitions:
+
+| Version | OpenSSL | Bundler |
+|---------|---------|---------|
+| 1.8.7-p374 | 1.0.2u | 1.17.3 |
+| 1.9.3-p551 | 1.0.2u | 1.17.3 |
+| 2.3.3 | 1.0.2u | 1.17.3 |
+| 2.3.8 | 1.0.2u | 1.17.3 |
+| 2.5.9 | 1.1.1w | 1.17.3 and 2.3.27 |
+| 2.7.8 | 1.1.1w | 1.17.3 and 2.4.22 |
+
+Modern OpenSSL dropped the APIs these Rubies need, so each definition brings its own. On
+macOS the OpenSSL 1.0 builds come from [basecamp/homebrew-dev](https://github.com/basecamp/homebrew-dev);
+everywhere else OpenSSL is compiled from source into the Ruby's own prefix, so nothing
+lands system-wide and nothing is shared between versions.
+
+### Prerequisites
+
+**macOS:** Xcode command line tools and [Homebrew](https://brew.sh).
+
+**Ubuntu:**
+
+```bash
+sudo apt-get install -y autoconf bison build-essential curl git libdb-dev libffi-dev \
+  libgdbm-dev libgmp-dev libncurses5-dev libreadline-dev libssl-dev libyaml-dev \
+  patch uuid-dev zlib1g-dev
+```
+
+**Arch / Omarchy:**
+
+```bash
+sudo pacman -S --needed autoconf base-devel bison git gmp libffi libyaml openssl patch readline zlib
+```
 
 ### mise
 
 ```bash
 git clone https://github.com/basecamp/ruby-dev
 
-RUBY_BUILD_DEFINITIONS="~/path/to/ruby-dev" mise use ruby@1.8.7-p374
+export RUBY_BUILD_DEFINITIONS="$PWD/ruby-dev"
+mise install ruby@1.8.7
+```
+
+`RUBY_BUILD_DEFINITIONS` must be an absolute path and must be set for every command that
+builds a Ruby — mise shells out to ruby-build, which reads it from the environment. See
+[the warning below](#a-warning-that-applies-to-all-of-the-above); a bad path fails silently.
+
+Fuzzy versions resolve to the definitions here, so `ruby@1.8.7` gets you `1.8.7-p374` and
+`ruby@2.3` gets you `2.3.8`.
+
+To pin a project to one:
+
+```bash
+export RUBY_BUILD_DEFINITIONS="/absolute/path/to/ruby-dev"
+mise use ruby@1.8.7-p374
 ```
 
 ### rbenv
 
+Same idea — point `RUBY_BUILD_DEFINITIONS` at the checkout:
+
 ```bash
 git clone https://github.com/basecamp/ruby-dev
 
-RUBY_BUILD_DEFINITIONS="./ruby-dev" rbenv install 1.8.7-p374
+RUBY_BUILD_DEFINITIONS="$PWD/ruby-dev" rbenv install 1.8.7-p374
 ```
 
-or
+Or skip the environment variable and hand ruby-build the definition file directly:
+
 ```bash
 git clone https://github.com/basecamp/ruby-dev
 cd ruby-dev
 rbenv install ./1.8.7-p374
 ```
 
-or
+Or grab a single definition without cloning:
+
 ```bash
 curl -O https://raw.githubusercontent.com/basecamp/ruby-dev/main/1.8.7-p374
 rbenv install ./1.8.7-p374
 ```
 
 (Note the leading `./` - this is a path to a specific build definition for ruby-build, not a version specific for it to look for among its built-in definitions.)
+
+### ruby-build on its own
+
+You don't need a version manager. ruby-build installs a Ruby into any prefix you name:
+
+```bash
+git clone https://github.com/basecamp/ruby-dev
+cd ruby-dev
+
+ruby-build ./1.8.7-p374 ~/.rubies/1.8.7-p374
+~/.rubies/1.8.7-p374/bin/ruby --version
+```
+
+`RUBY_BUILD_DEFINITIONS` works here too, and is the better choice when something else is
+invoking ruby-build on your behalf:
+
+```bash
+RUBY_BUILD_DEFINITIONS="$PWD" ruby-build 1.8.7-p374 ~/.rubies/1.8.7-p374
+```
+
+This is also the form to use with chruby or any other manager that just wants a directory
+of Rubies — build into its search path (`~/.rubies` for chruby) and it'll pick them up.
+
+### A warning that applies to all of the above
+
+Whichever tool you use, the definitions are only found if ruby-build can actually see them:
+
+- **`RUBY_BUILD_DEFINITIONS` must be an absolute path.** Build tools run from temporary
+  working directories, so a relative path usually resolves to nowhere.
+- **A wrong path fails silently.** ruby-build appends its own bundled definition directory
+  to the search list and takes the first match, so a typo'd or unexpanded path doesn't
+  error — it quietly builds the *upstream* definition instead, which is exactly the one
+  that fails on a modern compiler. Confusing compiler errors are the usual symptom.
+- **`"~/path/to/ruby-dev"` does not expand.** The tilde is literal inside double quotes.
+  Use `"$HOME/path/to/ruby-dev"` or leave it unquoted.
+
+If a build fails in a way that looks nothing like the notes in this repo, check that the
+definition was actually picked up before debugging the compiler error.
+
+### Testing
+
+`test/build` builds definitions in throwaway Docker containers, so a clean-machine build
+is checked without touching your own toolchain.
+
+```bash
+test/build arch 1.8.7-p374     # one version on one platform
+test/build ubuntu-noble all    # every version on one platform
+test/build all                 # everything
+```
+
+Builds run concurrently, so results stream in out of order and a sorted summary with any
+failure logs is printed at the end. Tune the load with `JOBS` (containers at a time,
+defaults to cores/4) and `MAKE_JOBS` (`make -j` inside each, defaults to 4):
+
+```bash
+JOBS=4 MAKE_JOBS=8 test/build all
+```
diff --git a/test/build b/test/build
index a7d4ed2..ec87675 100755
--- a/test/build
+++ b/test/build
@@ -6,6 +6,15 @@ cd "$(dirname "$0")/.."
 PLATFORMS="ubuntu-noble arch"
 VERSIONS=$(ls -1 [0-9]* 2>/dev/null | sort -rV)
 
+# Builds run concurrently. JOBS caps how many containers are in flight; MAKE_JOBS caps
+# make parallelism inside each one. The product is what actually hits the CPU, so the
+# defaults aim for a mild oversubscribe rather than JOBS x nproc meltdown.
+CORES=$(nproc 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || echo 4)
+JOBS=${JOBS:-$(( CORES / 4 ))}
+(( JOBS < 1 )) && JOBS=1
+(( JOBS > 12 )) && JOBS=12
+MAKE_JOBS=${MAKE_JOBS:-4}
+
 # Use multi-platform builder if available (faster cross-arch builds)
 BUILDER=""
 if docker buildx ls 2>/dev/null | grep -A2 "^multiarch" | grep -q "running"; then
@@ -24,6 +33,13 @@ Examples:
   test/build arch all              # Test all versions on Arch
   test/build all 2.7.8             # Test Ruby 2.7.8 on all platforms
   test/build all                   # Test everything
+
+Environment:
+  JOBS=$JOBS         Containers built concurrently
+  MAKE_JOBS=$MAKE_JOBS      make -j inside each container
+
+Results stream in as builds finish, so they arrive out of order. A sorted
+summary is printed at the end.
 EOF
   exit 1
 }
@@ -90,8 +106,6 @@ test_ruby() {
 
   [[ -n "$target_platform" ]] && platform_flag="--platform $target_platform"
 
-  printf "  %-14s %-14s " "$platform" "$version"
-
   # Use version-specific verify script if it exists, otherwise use defaults
   local verify_script
   if [[ -f "test/verify/$version" ]]; then
@@ -108,15 +122,23 @@ test_ruby() {
     $verify_script
   "
 
-  if output=$(docker run --rm $platform_flag "$image" bash -c "$build_script" 2>&1); then
+  local started=$SECONDS output elapsed
+  if output=$(docker run --rm $platform_flag -e MAKE_OPTS="-j${MAKE_JOBS}" \
+                "$image" bash -c "$build_script" 2>&1); then
+    elapsed=$(( SECONDS - started ))
     ruby_version=$(echo "$output" | grep -o 'ruby [0-9].*\]' | tail -1)
-    echo "✓  $ruby_version"
-    return 0
+    # One printf so concurrent jobs can't interleave mid-line.
+    printf '  %-14s %-14s ✓  %-62s %4ds\n' "$platform" "$version" "$ruby_version" "$elapsed"
+    echo "pass" > "$RESULTS/$platform.$version.status"
   else
-    echo "✗  FAILED"
-    echo "$output" | tail -20
-    return 1
+    elapsed=$(( SECONDS - started ))
+    printf '  %-14s %-14s ✗  %-62s %4ds\n' "$platform" "$version" "FAILED" "$elapsed"
+    echo "fail" > "$RESULTS/$platform.$version.status"
+    # Keep the log for the end-of-run report rather than interleaving it with
+    # other jobs' output as it happens.
+    echo "$output" | tail -30 > "$RESULTS/$platform.$version.log"
   fi
+  return 0
 }
 
 [[ $# -lt 1 ]] && usage
@@ -142,15 +164,44 @@ for p in $platforms; do
   build_image "$p"
 done
 
-# Run tests
+# Run tests, up to $JOBS at a time
+RESULTS=$(mktemp -d)
+trap 'rm -rf "$RESULTS"' EXIT
+
+total=0
+for p in $platforms; do for v in $versions; do total=$(( total + 1 )); done; done
+
+echo ""
+echo "Running $total build(s), $JOBS at a time (make -j$MAKE_JOBS each)..."
 echo ""
+
+for p in $platforms; do
+  for v in $versions; do
+    # Throttle: wait for a slot before launching the next build. `wait -n` would be
+    # tidier but needs Bash 4.3, and macOS still ships 3.2 as /bin/bash — there it
+    # fails instantly and spins the loop on a core. Poll instead; builds run for
+    # minutes, so a second of latency costs nothing.
+    while (( $(jobs -rp | wc -l) >= JOBS )); do
+      sleep 1
+    done
+    test_ruby "$p" "$v" &
+  done
+done
+wait
+
+# Summarize in stable order, since results streamed in as they finished.
 failed=0
 for p in $platforms; do
   for v in $versions; do
-    test_ruby "$p" "$v" || ((failed++)) || true
+    if [[ $(cat "$RESULTS/$p.$v.status" 2>/dev/null) != "pass" ]]; then
+      failed=$(( failed + 1 ))
+      echo ""
+      echo "=== $p $v failed ==="
+      cat "$RESULTS/$p.$v.log" 2>/dev/null || echo "(no output captured)"
+    fi
   done
 done
 
 echo ""
-[[ $failed -eq 0 ]] && echo "All tests passed." || echo "$failed test(s) failed."
+[[ $failed -eq 0 ]] && echo "All $total tests passed." || echo "$failed of $total test(s) failed."
 exit $failed