diff --git a/src/grammar/generic/SqlLexer.g4 b/src/grammar/generic/SqlLexer.g4
index a1745b1..d0192d1 100644
--- a/src/grammar/generic/SqlLexer.g4
+++ b/src/grammar/generic/SqlLexer.g4
@@ -23,6 +23,11 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */
 
+/**
+ * This file is an adaptation of antlr's grammars-v4/sql/mysql/Positive-Technologies/MySqlLexer.g4 grammar.
+ * Reference: https://github.com/antlr/grammars-v4/blob/master/sql/mysql/Positive-Technologies/MySqlLexer.g4
+ */
+
 lexer grammar SqlLexer;
 
 channels { MYSQLCOMMENT, ERRORCHANNEL }
diff --git a/src/grammar/generic/SqlParser.g4 b/src/grammar/generic/SqlParser.g4
index 2ca69c3..35038af 100644
--- a/src/grammar/generic/SqlParser.g4
+++ b/src/grammar/generic/SqlParser.g4
@@ -23,6 +23,11 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */
 
+/**
+ * This file is an adaptation of antlr's grammars-v4/sql/mysql/Positive-Technologies/MySqlParser.g4 grammar.
+ * Reference: https://github.com/antlr/grammars-v4/blob/master/sql/mysql/Positive-Technologies/MySqlParser.g4
+ */
+
 parser grammar SqlParser;
 
 options { tokenVocab=SqlLexer; }
diff --git a/src/grammar/hive/HiveSqlLexer.g4 b/src/grammar/hive/HiveSqlLexer.g4
index 3a9a22b..830aae0 100644
--- a/src/grammar/hive/HiveSqlLexer.g4
+++ b/src/grammar/hive/HiveSqlLexer.g4
@@ -14,6 +14,12 @@
    See the License for the specific language governing permissions and
    limitations under the License.
 */
+
+/**
+ * This file is an adaptation of antlr's grammars-v4/sql/hive/v4/HiveLexer.g4 grammar.
+ * Reference: https://github.com/antlr/grammars-v4/blob/master/sql/hive/v4/HiveLexer.g4
+ */
+
 lexer grammar HiveSqlLexer;
 
 // unsupported option caseInsensitive in antlr4@4.9
diff --git a/src/grammar/hive/HiveSqlParser.g4 b/src/grammar/hive/HiveSqlParser.g4
index 4db1e35..7d66629 100644
--- a/src/grammar/hive/HiveSqlParser.g4
+++ b/src/grammar/hive/HiveSqlParser.g4
@@ -14,6 +14,12 @@
    See the License for the specific language governing permissions and
    limitations under the License.
 */
+
+/**
+ * This file is an adaptation of antlr/grammars-v4's sql/hive/v4/HiveParser.g4 grammar.
+ * Reference: https://github.com/antlr/grammars-v4/blob/master/sql/hive/v4/HiveParser.g4
+ */
+
 parser grammar HiveSqlParser;
 
 options
diff --git a/src/grammar/pgsql/PostgreSQLLexer.g4 b/src/grammar/pgsql/PostgreSQLLexer.g4
index bef5c42..8734112 100644
--- a/src/grammar/pgsql/PostgreSQLLexer.g4
+++ b/src/grammar/pgsql/PostgreSQLLexer.g4
@@ -1,8 +1,3 @@
-/*
- based on
- https://github.com/tunnelvisionlabs/antlr4-grammar-postgresql/blob/master/src/com/tunnelvisionlabs/postgresql/PostgreSqlLexer.g4
- */
-
 /*
  * [The "MIT license"] Copyright (C) 2014 Sam Harwell, Tunnel Vision Laboratories, LLC
  * 
@@ -23,8 +18,16 @@
  * other dealings in this Software without prior written authorization from Tunnel Vision
  * Laboratories, LLC.
  */
+
+/**
+ * This file is an adaptation of tunnelvisionlabs's antlr4-grammar-postgresql/src/com/tunnelvisionlabs/postgresql/PostgreSqlLexer.g4 grammar.
+ * Reference: https://github.com/tunnelvisionlabs/antlr4-grammar-postgresql/blob/master/src/com/tunnelvisionlabs/postgresql/PostgreSqlLexer.g4
+ */
+
 lexer grammar PostgreSQLLexer;
-/* Reference: http://www.postgresql.org/docs/9.3/static/sql-syntax-lexical.html
+
+/**
+ * Reference Doc: http://www.postgresql.org/docs/9.3/static/sql-syntax-lexical.html
  */
 
 options {
diff --git a/src/grammar/plsql/PlSqlLexer.g4 b/src/grammar/plsql/PlSqlLexer.g4
index 0774471..36a8c01 100644
--- a/src/grammar/plsql/PlSqlLexer.g4
+++ b/src/grammar/plsql/PlSqlLexer.g4
@@ -18,6 +18,11 @@
  * limitations under the License.
  */
 
+/**
+ * This file is an adaptation of antlr's grammars-v4/sql/plsql/PlSqlLexer.g4 grammar.
+ * Reference: https://github.com/antlr/grammars-v4/blob/master/sql/plsql/PlSqlLexer.g4
+ */
+
 lexer grammar PlSqlLexer;
 
 options {
diff --git a/src/grammar/plsql/PlSqlParser.g4 b/src/grammar/plsql/PlSqlParser.g4
index 2a4cf44..02ca62a 100644
--- a/src/grammar/plsql/PlSqlParser.g4
+++ b/src/grammar/plsql/PlSqlParser.g4
@@ -18,6 +18,11 @@
  * limitations under the License.
  */
 
+/**
+ * This file is an adaptation of antlr's grammars-v4/sql/plsql/PlSqlParser.g4 grammar.
+ * Reference: https://github.com/antlr/grammars-v4/blob/master/sql/plsql/PlSqlParser.g4
+ */
+
 parser grammar PlSqlParser;
 
 options {
diff --git a/src/grammar/spark/SparkSqlLexer.g4 b/src/grammar/spark/SparkSqlLexer.g4
index 8793c10..3c4dcda 100644
--- a/src/grammar/spark/SparkSqlLexer.g4
+++ b/src/grammar/spark/SparkSqlLexer.g4
@@ -1,5 +1,3 @@
-// Grammar file from: https://github.com/apache/spark/blob/master/sql/api/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBaseLexer.g4
-
 /*
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -13,7 +11,11 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  *
- * This file is an adaptation of Presto's presto-parser/src/main/antlr4/com/facebook/presto/sql/parser/SqlBase.g4 grammar.
+ */
+
+/**
+ * This file is an adaptation of spark's spark/sql/api/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBaseLexer.g4 grammar.
+ * Reference: https://github.com/apache/spark/blob/master/sql/api/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBaseLexer.g4
  */
 
 lexer grammar SparkSqlLexer;
diff --git a/src/grammar/spark/SparkSqlParser.g4 b/src/grammar/spark/SparkSqlParser.g4
index 5542bca..f68dd61 100644
--- a/src/grammar/spark/SparkSqlParser.g4
+++ b/src/grammar/spark/SparkSqlParser.g4
@@ -1,5 +1,3 @@
-// Grammar file from: https://github.com/apache/spark/blob/master/sql/api/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBaseParser.g4
-
 /*
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -13,7 +11,11 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  *
- * This file is an adaptation of Presto's presto-parser/src/main/antlr4/com/facebook/presto/sql/parser/SqlBase.g4 grammar.
+ */
+
+/**
+ * This file is an adaptation of spark's spark/sql/api/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBaseParser.g4 grammar.
+ * Reference: https://github.com/apache/spark/blob/master/sql/api/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBaseParser.g4
  */
 
 parser grammar SparkSqlParser;
diff --git a/src/grammar/trinosql/TrinoSql.g4 b/src/grammar/trinosql/TrinoSql.g4
index a3f6f58..302ab25 100644
--- a/src/grammar/trinosql/TrinoSql.g4
+++ b/src/grammar/trinosql/TrinoSql.g4
@@ -1,3 +1,22 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * This file is an adaptation of trino's trino/core/trino-parser/src/main/antlr4/io/trino/sql/parser/SqlBase.g4 grammar.
+ * Reference: https://github.com/trinodb/trino/blob/385/core/trino-parser/src/main/antlr4/io/trino/sql/parser/SqlBase.g4
+ */
+
 grammar TrinoSql;
 
 tokens {