diff --git a/doc/03-cli.md b/doc/03-cli.md
index 6722fa8b3..e9b78e593 100644
--- a/doc/03-cli.md
+++ b/doc/03-cli.md
@@ -736,6 +736,8 @@ See the [Config](06-config.md) chapter for valid configuration options.
   instead of relative.
 * **--json:** JSON decode the setting value, to be used with `extra.*` keys.
 * **--merge:** Merge the setting value with the current value, to be used with `extra.*` keys in combination with `--json`.
+* **--append:** When adding a repository, append it (lowest priority) to the existing ones instead of prepending it (highest priority).
+* **--source:** Display where the config value is loaded from.
 
 ### Modifying Repositories
 
diff --git a/src/Composer/Command/ConfigCommand.php b/src/Composer/Command/ConfigCommand.php
index 03ebd9682..e9a22ab46 100644
--- a/src/Composer/Command/ConfigCommand.php
+++ b/src/Composer/Command/ConfigCommand.php
@@ -77,6 +77,7 @@ class ConfigCommand extends BaseCommand
                 new InputOption('json', 'j', InputOption::VALUE_NONE, 'JSON decode the setting value, to be used with extra.* keys'),
                 new InputOption('merge', 'm', InputOption::VALUE_NONE, 'Merge the setting value with the current value, to be used with extra.* keys in combination with --json'),
                 new InputOption('append', null, InputOption::VALUE_NONE, 'When adding a repository, append it (lowest priority) to the existing ones instead of prepending it (highest priority)'),
+                new InputOption('source', null, InputOption::VALUE_NONE, 'Display where the config value is loaded from'),
                 new InputArgument('setting-key', null, 'Setting key'),
                 new InputArgument('setting-value', InputArgument::IS_ARRAY, 'Setting value'),
             ))
@@ -234,13 +235,13 @@ EOT
         }
 
         if (!$input->getOption('global')) {
-            $this->config->merge($this->configFile->read());
-            $this->config->merge(array('config' => $this->authConfigFile->exists() ? $this->authConfigFile->read() : array()));
+            $this->config->merge($this->configFile->read(), $this->configFile->getPath());
+            $this->config->merge(array('config' => $this->authConfigFile->exists() ? $this->authConfigFile->read() : array()), $this->authConfigFile->getPath());
         }
 
         // List the configuration of the file settings
         if ($input->getOption('list')) {
-            $this->listConfiguration($this->config->all(), $this->config->raw(), $output);
+            $this->listConfiguration($this->config->all(), $this->config->raw(), $output, null, (bool) $input->getOption('source'));
 
             return 0;
         }
@@ -305,7 +306,12 @@ EOT
                 $value = json_encode($value);
             }
 
-            $this->getIO()->write($value, true, IOInterface::QUIET);
+            $sourceOfConfigValue = '';
+            if ($input->getOption('source')) {
+                $sourceOfConfigValue = ' (' . $this->config->getSourceOfValue($settingKey) . ')';
+            }
+
+            $this->getIO()->write($value . $sourceOfConfigValue, true, IOInterface::QUIET);
 
             return 0;
         }
@@ -823,10 +829,11 @@ EOT
      * @param array<array|bool|string> $contents
      * @param array<array|string>      $rawContents
      * @param string|null              $k
+     * @param bool                     $showSource
      *
      * @return void
      */
-    protected function listConfiguration(array $contents, array $rawContents, OutputInterface $output, $k = null)
+    protected function listConfiguration(array $contents, array $rawContents, OutputInterface $output, $k = null, $showSource = false)
     {
         $origK = $k;
         $io = $this->getIO();
@@ -839,7 +846,7 @@ EOT
 
             if (is_array($value) && (!is_numeric(key($value)) || ($key === 'repositories' && null === $k))) {
                 $k .= preg_replace('{^config\.}', '', $key . '.');
-                $this->listConfiguration($value, $rawVal, $output, $k);
+                $this->listConfiguration($value, $rawVal, $output, $k, $showSource);
                 $k = $origK;
 
                 continue;
@@ -857,10 +864,14 @@ EOT
                 $value = var_export($value, true);
             }
 
+            $source = '';
+            if ($showSource) {
+                $source = ' (' . $this->config->getSourceOfValue($k . $key) . ')';
+            }
             if (is_string($rawVal) && $rawVal != $value) {
-                $io->write('[<comment>' . $k . $key . '</comment>] <info>' . $rawVal . ' (' . $value . ')</info>', true, IOInterface::QUIET);
+                $io->write('[<comment>' . $k . $key . '</comment>] <info>' . $rawVal . ' (' . $value . ')</info>' . $source, true, IOInterface::QUIET);
             } else {
-                $io->write('[<comment>' . $k . $key . '</comment>] <info>' . $value . '</info>', true, IOInterface::QUIET);
+                $io->write('[<comment>' . $k . $key . '</comment>] <info>' . $value . '</info>' . $source, true, IOInterface::QUIET);
             }
         }
     }
diff --git a/src/Composer/Command/CreateProjectCommand.php b/src/Composer/Command/CreateProjectCommand.php
index 0cf206a53..233ed1a2e 100644
--- a/src/Composer/Command/CreateProjectCommand.php
+++ b/src/Composer/Command/CreateProjectCommand.php
@@ -354,7 +354,7 @@ EOT
     protected function installRootPackage(IOInterface $io, Config $config, $packageName, PlatformRequirementFilterInterface $platformRequirementFilter, $directory = null, $packageVersion = null, $stability = 'stable', $preferSource = false, $preferDist = false, $installDevPackages = false, array $repositories = null, $disablePlugins = false, $noScripts = false, $noProgress = false, $secureHttp = true)
     {
         if (!$secureHttp) {
-            $config->merge(array('config' => array('secure-http' => false)));
+            $config->merge(array('config' => array('secure-http' => false)), Config::SOURCE_COMMAND);
         }
 
         $parser = new VersionParser();
diff --git a/src/Composer/Command/DiagnoseCommand.php b/src/Composer/Command/DiagnoseCommand.php
index 8e4e617cc..80c951833 100644
--- a/src/Composer/Command/DiagnoseCommand.php
+++ b/src/Composer/Command/DiagnoseCommand.php
@@ -90,7 +90,7 @@ EOT
             $config = Factory::createConfig();
         }
 
-        $config->merge(array('config' => array('secure-http' => false)));
+        $config->merge(array('config' => array('secure-http' => false)), Config::SOURCE_COMMAND);
         $config->prohibitUrlByConfig('http://repo.packagist.org', new NullIO);
 
         $this->httpDownloader = Factory::createHttpDownloader($io, $config);
diff --git a/src/Composer/Config.php b/src/Composer/Config.php
index 0cbbf27dd..9d879013a 100644
--- a/src/Composer/Config.php
+++ b/src/Composer/Config.php
@@ -23,6 +23,10 @@ use Composer\Util\ProcessExecutor;
  */
 class Config
 {
+    const SOURCE_DEFAULT = 'default';
+    const SOURCE_COMMAND = 'command';
+    const SOURCE_UNKNOWN = 'unknown';
+
     const RELATIVE_PATHS = 1;
 
     /** @var array<string, mixed> */
@@ -100,6 +104,8 @@ class Config
     private $useEnvironment;
     /** @var array<string, true> */
     private $warnedHosts = array();
+    /** @var array<string, string> */
+    private $sourceOfConfigValue = array();
 
     /**
      * @param bool   $useEnvironment Use COMPOSER_ environment variables to replace config settings
@@ -112,6 +118,14 @@ class Config
         $this->repositories = static::$defaultRepositories;
         $this->useEnvironment = (bool) $useEnvironment;
         $this->baseDir = $baseDir;
+
+        foreach ($this->config as $configKey => $configValue) {
+            $this->setSourceOfConfigValue($configValue, $configKey, self::SOURCE_DEFAULT);
+        }
+
+        foreach ($this->repositories as $configKey => $configValue) {
+            $this->setSourceOfConfigValue($configValue, 'repositories.' . $configKey, self::SOURCE_DEFAULT);
+        }
     }
 
     /**
@@ -150,18 +164,21 @@ class Config
      * Merges new config values with the existing ones (overriding)
      *
      * @param array<string, mixed> $config
+     * @param string $source
      *
      * @return void
      */
-    public function merge($config)
+    public function merge($config, $source = self::SOURCE_UNKNOWN)
     {
         // override defaults with given config
         if (!empty($config['config']) && is_array($config['config'])) {
             foreach ($config['config'] as $key => $val) {
                 if (in_array($key, array('bitbucket-oauth', 'github-oauth', 'gitlab-oauth', 'gitlab-token', 'http-basic', 'bearer')) && isset($this->config[$key])) {
                     $this->config[$key] = array_merge($this->config[$key], $val);
+                    $this->setSourceOfConfigValue($val, $key, $source);
                 } elseif (in_array($key, array('gitlab-domains', 'github-domains')) && isset($this->config[$key])) {
                     $this->config[$key] = array_unique(array_merge($this->config[$key], $val));
+                    $this->setSourceOfConfigValue($val, $key, $source);
                 } elseif ('preferred-install' === $key && isset($this->config[$key])) {
                     if (is_array($val) || is_array($this->config[$key])) {
                         if (is_string($val)) {
@@ -169,8 +186,10 @@ class Config
                         }
                         if (is_string($this->config[$key])) {
                             $this->config[$key] = array('*' => $this->config[$key]);
+                            $this->sourceOfConfigValue[$key . '*'] = $source;
                         }
                         $this->config[$key] = array_merge($this->config[$key], $val);
+                        $this->setSourceOfConfigValue($val, $key, $source);
                         // the full match pattern needs to be last
                         if (isset($this->config[$key]['*'])) {
                             $wildcard = $this->config[$key]['*'];
@@ -179,9 +198,11 @@ class Config
                         }
                     } else {
                         $this->config[$key] = $val;
+                        $this->setSourceOfConfigValue($val, $key, $source);
                     }
                 } else {
                     $this->config[$key] = $val;
+                    $this->setSourceOfConfigValue($val, $key, $source);
                 }
             }
         }
@@ -210,11 +231,14 @@ class Config
                 // store repo
                 if (is_int($name)) {
                     $this->repositories[] = $repository;
+                    $this->setSourceOfConfigValue($repository, 'repositories.' . array_search($repository, $this->repositories, true), $source);
                 } else {
                     if ($name === 'packagist') { // BC support for default "packagist" named repo
                         $this->repositories[$name . '.org'] = $repository;
+                        $this->setSourceOfConfigValue($repository, 'repositories.' . $name . '.org', $source);
                     } else {
                         $this->repositories[$name] = $repository;
+                        $this->setSourceOfConfigValue($repository, 'repositories.' . $name, $source);
                     }
                 }
             }
@@ -257,6 +281,10 @@ class Config
                 $env = 'COMPOSER_' . strtoupper(strtr($key, '-', '_'));
 
                 $val = $this->getComposerEnv($env);
+                if ($val !== false) {
+                    $this->setSourceOfConfigValue($val, $key, $env);
+                }
+
                 $val = rtrim((string) $this->process(false !== $val ? $val : $this->config[$key], $flags), '/\\');
                 $val = Platform::expandPath($val);
 
@@ -275,6 +303,8 @@ class Config
                 $val = $this->getComposerEnv($env);
                 if (false === $val) {
                     $val = $this->config[$key];
+                } else {
+                    $this->setSourceOfConfigValue($val, $key, $env);
                 }
 
                 return $val !== 'false' && (bool) $val;
@@ -405,6 +435,35 @@ class Config
         return $all;
     }
 
+    /**
+     * @param string $key
+     * @return string
+     */
+    public function getSourceOfValue($key)
+    {
+        $this->get($key);
+
+        return isset($this->sourceOfConfigValue[$key]) ? $this->sourceOfConfigValue[$key] : self::SOURCE_UNKNOWN;
+    }
+
+    /**
+     * @param mixed  $configValue
+     * @param string $path
+     * @param string $source
+     *
+     * @return void
+     */
+    private function setSourceOfConfigValue($configValue, $path, $source)
+    {
+        $this->sourceOfConfigValue[$path] = $source;
+
+        if (is_array($configValue)) {
+            foreach ($configValue as $key => $value) {
+                $this->setSourceOfConfigValue($value, $path . '.' . $key, $source);
+            }
+        }
+    }
+
     /**
      * @return array<string, mixed[]>
      */
diff --git a/src/Composer/Factory.php b/src/Composer/Factory.php
index fdcee9034..6b5e38c03 100644
--- a/src/Composer/Factory.php
+++ b/src/Composer/Factory.php
@@ -186,7 +186,7 @@ class Factory
             'home' => $home,
             'cache-dir' => self::getCacheDir($home),
             'data-dir' => self::getDataDir($home),
-        )));
+        )), Config::SOURCE_DEFAULT);
 
         // load global config
         $file = new JsonFile($config->get('home').'/config.json');
@@ -194,7 +194,7 @@ class Factory
             if ($io && $io->isDebug()) {
                 $io->writeError('Loading config file ' . $file->getPath());
             }
-            $config->merge($file->read());
+            $config->merge($file->read(), $file->getPath());
         }
         $config->setConfigSource(new JsonConfigSource($file));
 
@@ -220,7 +220,7 @@ class Factory
             if ($io && $io->isDebug()) {
                 $io->writeError('Loading config file ' . $file->getPath());
             }
-            $config->merge(array('config' => $file->read()));
+            $config->merge(array('config' => $file->read()), $file->getPath());
         }
         $config->setAuthConfigSource(new JsonConfigSource($file, true));
 
@@ -236,7 +236,7 @@ class Factory
                 if ($io && $io->isDebug()) {
                     $io->writeError('Loading auth config from COMPOSER_AUTH');
                 }
-                $config->merge(array('config' => $authData));
+                $config->merge(array('config' => $authData), 'COMPOSER_AUTH');
             }
         }
 
@@ -309,6 +309,7 @@ class Factory
             $localConfig = static::getComposerFile();
         }
 
+        $localConfigSource = Config::SOURCE_UNKNOWN;
         if (is_string($localConfig)) {
             $composerFile = $localConfig;
 
@@ -340,11 +341,12 @@ class Factory
             }
 
             $localConfig = $file->read();
+            $localConfigSource = $file->getPath();
         }
 
         // Load config and override with local config/auth config
         $config = static::createConfig($io, $cwd);
-        $config->merge($localConfig);
+        $config->merge($localConfig, $localConfigSource);
         if (isset($composerFile)) {
             $io->writeError('Loading config file ' . $composerFile .' ('.realpath($composerFile).')', true, IOInterface::DEBUG);
             $config->setConfigSource(new JsonConfigSource(new JsonFile(realpath($composerFile), null, $io)));
@@ -352,7 +354,7 @@ class Factory
             $localAuthFile = new JsonFile(dirname(realpath($composerFile)) . '/auth.json', null, $io);
             if ($localAuthFile->exists()) {
                 $io->writeError('Loading config file ' . $localAuthFile->getPath(), true, IOInterface::DEBUG);
-                $config->merge(array('config' => $localAuthFile->read()));
+                $config->merge(array('config' => $localAuthFile->read()), $localAuthFile->getPath());
                 $config->setAuthConfigSource(new JsonConfigSource($localAuthFile, true));
             }
         }
diff --git a/tests/Composer/Test/ConfigTest.php b/tests/Composer/Test/ConfigTest.php
index ebc75ce04..06f655721 100644
--- a/tests/Composer/Test/ConfigTest.php
+++ b/tests/Composer/Test/ConfigTest.php
@@ -340,4 +340,26 @@ class ConfigTest extends TestCase
         $this->assertEquals(0, $config->get('htaccess-protect'));
         putenv('COMPOSER_HTACCESS_PROTECT');
     }
+
+    public function testGetSourceOfValue()
+    {
+        $config = new Config;
+
+        $this->assertSame(Config::SOURCE_DEFAULT, $config->getSourceOfValue('process-timeout'));
+
+        $config->merge(
+            array('config' => array('process-timeout' => 1)),
+            'phpunit-test'
+        );
+
+        $this->assertSame('phpunit-test', $config->getSourceOfValue('process-timeout'));
+    }
+
+    public function testGetSourceOfValueEnvVariables()
+    {
+        putenv('COMPOSER_HTACCESS_PROTECT=0');
+        $config = new Config;
+        $this->assertEquals('COMPOSER_HTACCESS_PROTECT', $config->getSourceOfValue('htaccess-protect'));
+        putenv('COMPOSER_HTACCESS_PROTECT');
+    }
 }