diff --git a/API.md b/API.md
index 12c77a3..88317c3 100644
--- a/API.md
+++ b/API.md
@@ -36,6 +36,13 @@ keys such as `{ in_folder: 'generated', with_name: '*_spec.rb' }`.
 
 ## File rules
 
+Every completed rule implementing `ArchUnit::Checkable` supports `.because(reason)`. Append it
+after all selectors and modifiers to get a distinct frozen rule of the same type, with a trimmed,
+immutable `rationale` string. Non-string or blank reasons raise `ArgumentError`. The original rule,
+violations, graph cache, and `check` options are unchanged. The shared result formatter adds
+`Because: ...` exactly once to failed framework-neutral, RSpec, and Minitest assertions, including
+failed negated RSpec expectations. Successful assertions and raw violation formatting omit it.
+
 Start with `ArchUnit.project_files(project_locator = nil)`; `ArchUnit.files` is its short alias.
 Without a locator, ArchUnitRuby searches upward for a `Gemfile` or gemspec.
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6c74829..6c9ff94 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ## 0.0.2 (unreleased)
 
+- Add immutable `.because(...)` explanations to every executable rule and shared assertion output.
+
 - Add opt-in debug inspection of project roots, graphs, and selected files on cold and cached checks.
 - Log passing metric values without repeating custom calculations.
 - Document verbosity, CI log artifacts, and existing colored violation reports.
diff --git a/README.md b/README.md
index a61bf9c..cd7189a 100644
--- a/README.md
+++ b/README.md
@@ -109,6 +109,23 @@ is required:
 
 ## 🚐 Setup
 
+### Explain why a rule exists
+
+Append `.because(...)` to a completed file, layer, slice, or metric rule:
+
+~~~ ruby
+rule = ArchUnit.project_files.in_folder('app/api/**')
+               .should_not.depend_on_files.in_folder('app/database/**')
+               .because('Keep persistence out of the API')
+
+expect(rule).to pass
+~~~
+
+The explanation appears once in a failed RSpec, Minitest, or `ArchUnit.assert_passes` assertion.
+It is available as `rule.rationale`. Calling `.because(...)` returns a distinct frozen rule;
+the original rule and direct `check` results are unchanged. Reasons must be non-blank strings.
+Add the explanation after all selectors and modifiers.
+
 ### Project Location
 
 The project locator is optional. With no argument, ArchUnitRuby searches from the current directory
diff --git a/lib/archunit/common/fluentapi/checkable.rb b/lib/archunit/common/fluentapi/checkable.rb
index b9796dd..2f1e8b9 100644
--- a/lib/archunit/common/fluentapi/checkable.rb
+++ b/lib/archunit/common/fluentapi/checkable.rb
@@ -11,6 +11,24 @@ module Common
     module FluentApi
       # The shared execution contract implemented by every terminal rule.
       module Checkable
+        # Optional normalized explanation, kept separate from violation values.
+        # @return [String, nil]
+        attr_reader :rationale
+
+        # Explains why this completed rule exists, without changing the original.
+        # Append this after all selectors and modifiers.
+        # @param reason [String] a non-blank human-readable explanation
+        # @return [Checkable] a distinct frozen rule of the same type
+        def because(reason)
+          unless reason.is_a?(String) && !reason.strip.empty?
+            raise ArgumentError, 'reason must be a non-empty String'
+          end
+
+          copy = dup
+          copy.instance_variable_set(:@rationale, reason.strip.freeze)
+          copy.freeze
+        end
+
         # Executes the rule and returns every architecture disagreement as structured data.
         # @param options [CheckOptions, nil]
         # @return [Array<ArchUnit::Violation>]
diff --git a/lib/archunit/testing.rb b/lib/archunit/testing.rb
index fbca7a4..42d9a3c 100644
--- a/lib/archunit/testing.rb
+++ b/lib/archunit/testing.rb
@@ -28,9 +28,7 @@ def result_for(rule, options = nil, expected_to_pass: true)
       end
 
       violations = rule.check(options)
-      return ResultFactory.from_violations(violations) if expected_to_pass
-
-      ResultFactory.from_violations(violations, expected_to_pass: false)
+      ResultFactory.from_violations(violations, expected_to_pass:, rationale: rule.rationale)
     end
   end
 
diff --git a/lib/archunit/testing/result_factory.rb b/lib/archunit/testing/result_factory.rb
index f5c99a7..b1df8e3 100644
--- a/lib/archunit/testing/result_factory.rb
+++ b/lib/archunit/testing/result_factory.rb
@@ -11,17 +11,29 @@ module Testing
     # Shapes a violation collection into one framework-neutral test result.
     class ResultFactory
       class << self
-        def from_violations(violations, color: nil, expected_to_pass: true)
+        def from_violations(violations, color: nil, expected_to_pass: true, rationale: nil)
           validate_violations(violations)
           validate_expected_to_pass(expected_to_pass)
           color = resolve_color(color)
-          return empty_result(color, expected_to_pass) if violations.empty?
-
-          failing_result(violations, color, expected_to_pass)
+          result = if violations.empty?
+                     empty_result(color, expected_to_pass)
+                   else
+                     failing_result(violations, color, expected_to_pass)
+                   end
+          explain_result(result, rationale)
         end
 
         private
 
+        def explain_result(result, rationale)
+          unless rationale.nil? || (rationale.is_a?(String) && !rationale.strip.empty?)
+            raise ArgumentError, 'rationale must be a non-empty String or nil'
+          end
+          return result unless rationale && result.failed?
+
+          TestResult.new(passed: false, message: "#{result.message}\n\nBecause: #{rationale.strip}")
+        end
+
         def passing_result(color)
           message = ColorUtils.green('No architecture violations found.', enabled: color)
           TestResult.new(passed: true, message:)
diff --git a/spec/common/fluentapi/rule_rationale_spec.rb b/spec/common/fluentapi/rule_rationale_spec.rb
new file mode 100644
index 0000000..d9389a9
--- /dev/null
+++ b/spec/common/fluentapi/rule_rationale_spec.rb
@@ -0,0 +1,149 @@
+# frozen_string_literal: true
+
+require 'fileutils'
+require 'minitest'
+require 'pathname'
+require 'tmpdir'
+
+RSpec.describe 'Rule rationale' do
+  around do |example|
+    Dir.mktmpdir('archunit-rationale') do |directory|
+      @root = Pathname.new(directory).realpath
+      @root.join('Gemfile').write('')
+      FileUtils.mkdir_p(@root.join('app/api'))
+      FileUtils.mkdir_p(@root.join('app/database'))
+      @root.join('app/api/request.rb').write("require_relative '../database/store'\n")
+      @root.join('app/database/store.rb').write('class Store; end')
+      ArchUnit.clear_graph_cache
+      example.run
+    ensure
+      ArchUnit.clear_graph_cache
+    end
+  end
+
+  let(:reason) { 'Keep persistence out of the API' }
+  let(:rule) do
+    ArchUnit.project_files(@root).in_folder('app/api/**')
+            .should_not.depend_on_files.in_folder('app/database/**')
+  end
+
+  # rubocop:disable-next Metrics/AbcSize, Metrics/MethodLength -- One fixture covers every terminal.
+  def all_terminals
+    files = ArchUnit.project_files(@root)
+    metrics = ArchUnit.metrics(@root)
+    slices = ArchUnit.project_slices(@root).defined_by('app/(**)/')
+    [
+      files.should.have_name('*.rb'),
+      files.should.have_no_cycles,
+      files.should_not.depend_on_files.in_folder('app/database/**'),
+      files.should_not.depend_on_external_modules.matching('json'),
+      files.should.adhere_to(->(_file) { true }, 'Ruby files'),
+      ArchUnit.project_layers(@root).layer('api').defined_by('app/api/**')
+              .where_layer('api').may_only_depend_on_layers,
+      slices.should_not.contain_dependency('api', 'database'),
+      slices.should.adhere_to_diagram(slices.to_plantuml),
+      metrics.count.classes.should_be_above(0),
+      metrics.count.classes.should_satisfy(->(value, _info) { value >= 0 }),
+      metrics.custom_metric('members', 'member count', ->(info) { info.methods.length })
+             .should_satisfy(->(value, _info) { value >= 0 }),
+      metrics.distance.not_in_zone_of_pain
+    ]
+  end
+
+  it 'supports every concrete terminal through one immutable common contract' do
+    terminals = all_terminals
+    classes = ObjectSpace.each_object(Class).select do |candidate|
+      candidate.name&.start_with?('ArchUnit::') && candidate.include?(ArchUnit::Checkable)
+    end
+    expect(terminals.map(&:class).uniq).to match_array(classes)
+
+    terminals.each do |original|
+      explained = original.because("  #{reason}  ")
+      expect(explained).to be_a(original.class)
+      expect(explained).not_to equal(original)
+      expect(explained).to be_frozen
+      expect(explained.rationale).to eq(reason)
+      expect(explained.rationale).to be_frozen
+      expect(original.rationale).to be_nil
+      expect(ArchUnit.format_violations(explained.check, color: false))
+        .to eq(ArchUnit.format_violations(original.check, color: false))
+    end
+  end
+
+  it 'owns its normalized string, supports replacement, and leaves caller values reusable' do
+    input = "  #{reason}  "
+    explained = rule.because(input)
+    input.replace('changed')
+    replacement = explained.because('A different explanation')
+
+    expect(explained.rationale).to eq(reason)
+    expect(replacement.rationale).to eq('A different explanation')
+    expect(rule.rationale).to be_nil
+  end
+
+  it 'rejects blank and non-string reasons' do
+    [nil, false, 42, :reason, '', " \t\n "].each do |invalid|
+      expect { rule.because(invalid) }.to raise_error(ArgumentError, /non-empty String/)
+    end
+  end
+
+  it 'does no analysis while copying a rule' do
+    rule.check
+    expect(Prism).not_to receive(:parse)
+    expect(ArchUnit::Extraction).not_to receive(:extract_graph)
+    explained = rule.because(reason)
+    expect(explained.rationale).to eq(reason)
+  end
+
+  it 'preserves graph cache identity and structured violations' do
+    violations = rule.check
+    graph = ArchUnit::Extraction.extract_graph(@root)
+    explained = rule.because(reason)
+    expect(explained.check).to eq(violations)
+    expect(ArchUnit::Extraction.extract_graph(@root)).to equal(graph)
+    expect(ArchUnit.format_violations(violations)).not_to include(reason)
+  end
+
+  it 'includes the reason exactly once in framework-neutral and RSpec failures' do
+    explained = rule.because(reason)
+    expect { ArchUnit.assert_passes(explained) }
+      .to raise_error(ArchUnit::AssertionFailure) do |error|
+        expect(error.message.scan(reason).length).to eq(1)
+      end
+    expect { expect(explained).to pass }
+      .to raise_error(RSpec::Expectations::ExpectationNotMetError) do |error|
+        expect(error.message.scan(reason).length).to eq(1)
+      end
+    expect(explained).not_to pass
+  end
+
+  it 'includes the reason exactly once in Minitest failures' do
+    ArchUnit::Testing::MinitestAdapter.install!
+    context = Class.new do
+      include Minitest::Assertions
+
+      attr_accessor :assertions
+    end.new
+    context.assertions = 0
+
+    expect { context.assert_passes(rule.because(reason)) }
+      .to raise_error(Minitest::Assertion) do |error|
+        expect(error.message.scan(reason).length).to eq(1)
+      end
+    expect(context.assertions).to eq(1)
+  end
+
+  it 'explains failed negated expectations but not successful assertions' do
+    passing = ArchUnit.project_files(@root).should.have_no_cycles.because(reason)
+    expect(passing).to pass
+    expect(ArchUnit.assert_passes(passing)).to be_nil
+    expect(ArchUnit::Testing.result_for(passing).message).not_to include(reason)
+    expect(ArchUnit::Testing.result_for(rule.because(reason), expected_to_pass: false).message)
+      .not_to include(reason)
+    expect { expect(passing).not_to pass }
+      .to raise_error(RSpec::Expectations::ExpectationNotMetError) do |error|
+        expect(error.message).to include('Expected architecture violations')
+        expect(error.message.scan(reason).length).to eq(1)
+      end
+  end
+end
diff --git a/spec/testing/assert_passes_spec.rb b/spec/testing/assert_passes_spec.rb
index a82d0e1..1d390e4 100644
--- a/spec/testing/assert_passes_spec.rb
+++ b/spec/testing/assert_passes_spec.rb
@@ -73,7 +73,7 @@ def create_file(relative_path, contents = '# fixture')
     rule = rule_class.new
 
     expect(ArchUnit::ResultFactory).to receive(:from_violations)
-      .with([violation]).and_call_original
+      .with([violation], expected_to_pass: true, rationale: nil).and_call_original
     expect { ArchUnit.assert_passes(rule) }.to raise_error(ArchUnit::AssertionFailure)
   end
 
diff --git a/spec/testing/result_factory_spec.rb b/spec/testing/result_factory_spec.rb
index cfbba62..c4c9452 100644
--- a/spec/testing/result_factory_spec.rb
+++ b/spec/testing/result_factory_spec.rb
@@ -74,4 +74,11 @@
       described_class.from_violations([violation], color: false).message
     )
   end
+
+  it 'validates optional rationale independently of the result' do
+    ['', '   ', false, 42].each do |invalid|
+      expect { described_class.from_violations([], rationale: invalid) }
+        .to raise_error(ArgumentError, /rationale/)
+    end
+  end
 end