diff --git a/PythonicGcodeMachine/Gcode/Rs274/Ast.py b/PythonicGcodeMachine/Gcode/Rs274/Ast.py
index 6cc4493e35cf1d3e493c1a13ed41b3368b05fb1c..e3df3e62bd0d49ff57123bd151e1f3d912ed235e 100644
--- a/PythonicGcodeMachine/Gcode/Rs274/Ast.py
+++ b/PythonicGcodeMachine/Gcode/Rs274/Ast.py
@@ -382,15 +382,26 @@ class Line(MachineMixin):
                 yield item
 
     def iter_on_gm_word(self):
-        for item in self:
-            if isinstance(item, Word) and item.is_gm_gcode:
+        for item in self.iter_on_word():
+            if item.is_gm_gcode:
                 yield item
 
     def iter_on_x_word(self):
-        for item in self:
-            if isinstance(item, Word) and not item.is_gm_gcode:
+        for item in self.iter_on_word():
+            if not item.is_gm_gcode:
+                yield item
+
+    def iter_on_letter(self, letters):
+        for item in self.iter_on_word():
+            if item.letter in letters:
                 yield item
 
+    def iter_on_g_word(self):
+        return self.iter_on_letter('G')
+
+    def iter_on_m_word(self):
+        return self.iter_on_letter('M')
+
     def iter_on_setting(self):
         for item in self:
             if isinstance(item, ParameterSetting):
@@ -421,6 +432,19 @@ class Line(MachineMixin):
 
     ##############################################
 
+    def check_modal_group(self):
+
+        modal_groups = {}
+        for item in self.iter_on_gm_word():
+            group = item.modal_group
+            modal_groups.setdefault(group, 0)
+            modal_groups[group] += 1
+        errors = [modal_group for modal_group, count in modal_groups.items() if count > 1]
+        if errors:
+            raise ValueError("Modal group errors {}".format(errors))
+
+    ##############################################
+
     def __repr__(self):
 
         items = []
@@ -514,6 +538,7 @@ class Word(LineItem):
 
     """Class to implement word"""
 
+    # Fixme: config ???
     LETTERS = (
         'A', 'B', 'C', 'D',
         'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', # 'N',
@@ -594,12 +619,20 @@ class Word(LineItem):
         if self._machine is None:
             raise NameError('Machine is not defined')
 
+    @property
+    def _machine_config(self):
+        self._check_machine()
+        return self._machine.config
+
     ##############################################
 
     @property
     def is_gm_gcode(self):
-        self._check_machine()
-        return self._machine.is_gm_word(self)
+        return self._machine_config.letters.is_gm_word(self)
+
+    @property
+    def is_axis_gcode(self):
+        return self._machine_config.letters.is_axis_word(self)
 
     ##############################################
 
diff --git a/PythonicGcodeMachine/Gcode/Rs274/Config.py b/PythonicGcodeMachine/Gcode/Rs274/Config.py
index 84c64a41af51684df9b00f2ee96d8640d245acf1..2663fc9daf243064b2599eb026e58ae281145180 100644
--- a/PythonicGcodeMachine/Gcode/Rs274/Config.py
+++ b/PythonicGcodeMachine/Gcode/Rs274/Config.py
@@ -36,9 +36,9 @@ __all__ = [
     'ExecutionOrder',
     'Gcode',
     'GcodeSet',
-    'Letters',
+    'LetterSet',
     'ModalGroup',
-    'ModalGroups',
+    'ModalGroupSet',
     'Parameter',
     'ParameterSet',
 ]
@@ -156,6 +156,11 @@ class Parameter(MeaningMixin):
     def default_value(self):
         return self._value
 
+    ##############################################
+
+    def __repr__(self):
+        return '#{0._index}: {0._meaning}'.format(self)
+
 ####################################################################################################
 
 class ParameterSet(YamlMixin, RstMixin):
@@ -210,12 +215,36 @@ class Letter(MeaningMixin):
         """G-code letter (table key)"""
         return self._letter
 
+    ##############################################
+
+    def __repr__(self):
+        return '#{0._letter}: {0._meaning}'.format(self)
+
 ####################################################################################################
 
-class Letters(YamlMixin, RstMixin):
+class LetterSet(YamlMixin, RstMixin):
 
     """Class for the table of letters."""
 
+    GM_LETTERS = 'GM'
+    AXIS_LETTERS = 'XYZABC' # 6-axis
+
+    ##############################################
+
+    def is_gm_letter(self, letter):
+        return letter in self.GM_LETTERS
+
+    def is_axis_letter(self, letter):
+        return letter in self.AXIS_LETTERS
+
+    ##############################################
+
+    def is_gm_word(self, word):
+        return self.is_gm_letter(word.letter)
+
+    def is_axis_word(self, word):
+        return self.is_axis_letter(word.letter)
+
     ##############################################
 
     def __init__(self, yaml_path):
@@ -251,7 +280,11 @@ class Gcode(MeaningMixin):
 
     ##############################################
 
-    def __init__(self, gcode, meaning, modal_group=None, execution_order=None):
+    def __init__(self, gcode, meaning,
+                 modal_group=None,
+                 execution_order=None,
+                 doc=None,
+    ):
 
         MeaningMixin.__init__(self, meaning)
         self._gcode = str(gcode)
@@ -259,6 +292,7 @@ class Gcode(MeaningMixin):
         # Those are set later due to the initialisation process
         self._modal_group = modal_group
         self._execution_order = execution_order
+        self._doc = doc
 
     ##############################################
 
@@ -279,6 +313,21 @@ class Gcode(MeaningMixin):
     def execution_order_index(self):
         return self._execution_order.index
 
+    @property
+    def doc(self):
+        return self._doc
+
+    ##############################################
+
+    def __str__(self):
+        return self._gcode
+
+    ##############################################
+
+    def convert_doc(self, format):
+        import pypandoc
+        return pypandoc.convert_text(self.doc, 'rst', format=format)
+
 ####################################################################################################
 
 class GcodeSet(YamlMixin, RstMixin):
@@ -295,6 +344,8 @@ class GcodeSet(YamlMixin, RstMixin):
             gcode = Gcode(gcode_txt, d['meaning'])
             self._gcodes[gcode_txt] = gcode
 
+        self._sorted_gcodes = None
+
     ##############################################
 
     def __len__(self):
@@ -311,11 +362,34 @@ class GcodeSet(YamlMixin, RstMixin):
 
     ##############################################
 
+    def _sort(self):
+
+        if self._sorted_gcodes is None:
+            items = list(self)
+            items.sort(key=lambda item: str(ord(item.gcode[0])*1000) + item.gcode[1:])
+            self._sorted_gcodes = items
+        return self._sorted_gcodes
+
+    ##############################################
+
     def sorted_iter(self):
+        return iter(self._sort())
 
-        items = list(self)
-        items.sort(key=lambda item: str(ord(item.gcode[0])*1000) + item.gcode[1:])
-        return items
+    ##############################################
+
+    def iter_on_slice(self, start, stop):
+
+        start_index = None
+        stop_index = None
+        for i, item in enumerate(self._sort()):
+            if item.gcode == start:
+                start_index = i
+            elif item.gcode == stop:
+                stop_index = i
+        if start_index > stop_index:
+            raise ValueError('{} > {}'.format(start, stop))
+
+        return iter(self._sorted_gcodes[start_index:stop_index+1])
 
     ##############################################
 
@@ -356,6 +430,11 @@ class ExecutionGroup(MeaningMixin):
         """Raw G-Codes list"""
         return self._raw_gcodes
 
+    ##############################################
+
+    def __str__(self):
+        return '#{0._index} Meaning: {0._meaning}'.format(self)
+
 ####################################################################################################
 
 class ExecutionOrder(YamlMixin, RstMixin):
@@ -445,9 +524,14 @@ class ModalGroup(MeaningMixin):
         """G-Codes list"""
         return self._gcodes
 
+    ##############################################
+
+    def __repr__(self):
+        return '#{0._index}: ({1}) Meaning: {0._meaning}'.format(self, ' '.join([str(gcode) for gcode in self._gcodes]))
+
 ####################################################################################################
 
-class ModalGroups(YamlMixin, RstMixin):
+class ModalGroupSet(YamlMixin, RstMixin):
 
     """Class for the table of modal groups."""
 
@@ -521,13 +605,54 @@ class Config:
 
         self._gcodes = GcodeSet(gcodes)
         self._execution_order = ExecutionOrder(execution_order, self._gcodes)
-        self._modal_groups = ModalGroups(modal_groups, self._gcodes)
+        self._modal_groups = ModalGroupSet(modal_groups, self._gcodes)
 
         # self._letters = str(letters)
         # self._parameters = str(parameters)
-        self._letters = Letters(letters)
+        self._letters = LetterSet(letters)
         self._parameters = ParameterSet(parameters)
 
+        self._load_doc()
+
+    ##############################################
+
+    def _load_doc(self):
+
+        from . import GcodeDoc as gcode_doc
+        for obj in gcode_doc.__dict__.values():
+            if isinstance(obj, type):
+                self._load_gcode_doc_cls(obj)
+
+    ##############################################
+
+    def _set_gcode_doc(self, gcode, cls):
+        rst_doc = cls.__doc__
+        rst_doc = rst_doc.replace('\n' + ' '*4, '\n')
+        self._gcodes[gcode]._doc = rst_doc
+
+    ##############################################
+
+    def _load_gcode_doc_cls(self, cls):
+
+        cls_name = cls.__name__
+        for letter in self._letters.GM_LETTERS:
+            cls_name = cls_name.replace('_' + letter, ' ' + letter)
+        cls_name = cls_name.replace('_to', '-')
+        cls_name = cls_name.replace('_', '.')
+        gcodes = cls_name.split(' ')
+        i = 0
+        while i < len(gcodes):
+            gcode = gcodes[i]
+            if gcode.endswith('-'):
+                start = gcode[:-1]
+                i += 1
+                stop = gcodes[i]
+                for _gcode in self._gcodes.iter_on_slice(start, stop):
+                    self._set_gcode_doc(str(_gcode), cls)
+            else:
+                self._set_gcode_doc(gcode, cls)
+            i += 1
+
     ##############################################
 
     @property
@@ -542,14 +667,14 @@ class Config:
 
     @property
     def letters(self):
-        """:class:`Letters` instance"""
+        """:class:`LetterSet` instance"""
         # if isinstance(self._letters, str):
-        #     self._letters = Letters(self._letters)
+        #     self._letters = LetterSet(self._letters)
         return self._letters
 
     @property
     def modal_groups(self):
-        """:class:`ModalGroups` instance"""
+        """:class:`ModalGroupSet` instance"""
         return self._modal_groups
 
     @property
diff --git a/PythonicGcodeMachine/Gcode/Rs274/Machine.py b/PythonicGcodeMachine/Gcode/Rs274/Machine.py
index 1864685cb6050496468cb011df905d26b7bbde0b..8e8f4f50521ae64d3c3acc74bba21de6586180a6 100644
--- a/PythonicGcodeMachine/Gcode/Rs274/Machine.py
+++ b/PythonicGcodeMachine/Gcode/Rs274/Machine.py
@@ -40,8 +40,6 @@ class GcodeMachine:
 
     PARSER_CLS = GcodeParser
 
-    GM_LETTERS = 'GM'
-
     ##############################################
 
     def __init__(self):
@@ -63,7 +61,7 @@ class GcodeMachine:
             letters=data_path.joinpath('rs274-word-starting-letter.yaml'),
             modal_groups=data_path.joinpath('rs274-modal-groups.yaml'),
             parameters=data_path.joinpath('rs274-default-parameter-file.yaml'),
-        )
+       )
 
     ##############################################
 
@@ -85,13 +83,3 @@ class GcodeMachine:
 
     def reset():
         pass
-
-    ##############################################
-
-    def is_gm_letter(self, letter):
-        return letter in self.GM_LETTERS
-
-    ##############################################
-
-    def is_gm_word(self, word):
-        return self.is_gm_letter(word.letter)
diff --git a/examples/gcode/annotate-gcode.py b/examples/gcode/annotate-gcode.py
index 0fbf7504963406188d4dd5f97394c1e6bb0b4c63..08a5dc1514a34ee60ea35a78dd62d5bcf56c9bc1 100644
--- a/examples/gcode/annotate-gcode.py
+++ b/examples/gcode/annotate-gcode.py
@@ -28,6 +28,8 @@
 #r#
 #r# * :mod:`PythonicGcodeMachine.Gcode.Rs274`
 #r# * :mod:`PythonicGcodeMachine.Gcode.Rs274.Ast`
+#r# * :mod:`PythonicGcodeMachine.Gcode.Rs274.Condig`
+#r# * :mod:`PythonicGcodeMachine.Gcode.Rs274.Machine`
 #r# * :mod:`PythonicGcodeMachine.Gcode.Rs274.Parser`
 
 ####################################################################################################
@@ -71,6 +73,7 @@ for line in program:
     print()
     # print(line.ansi_str()) # Fixme: pyterate
     print(str(line))
+    line.check_modal_group()
     for word in line.iter_on_word():
         if word.is_gm_gcode:
             margin = ' '*9
diff --git a/examples/gcode/query-gcode.py b/examples/gcode/query-gcode.py
new file mode 100644
index 0000000000000000000000000000000000000000..76d39474d8b58b5a5498371251e1fb2fe979f6bc
--- /dev/null
+++ b/examples/gcode/query-gcode.py
@@ -0,0 +1,74 @@
+#?##################################################################################################
+#?#
+#?# PythonicGcodeMachine - A Python G-code Toolkit
+#?# Copyright (C) 2018 Fabrice Salvaire
+#?#
+#?# This program is free software: you can redistribute it and/or modify
+#?# it under the terms of the GNU General Public License as published by
+#?# the Free Software Foundation, either version 3 of the License, or
+#?# (at your option) any later version.
+#?#
+#?# This program is distributed in the hope that it will be useful,
+#?# but WITHOUT ANY WARRANTY; without even the implied warranty of
+#?# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#?# GNU General Public License for more details.
+#?#
+#?# You should have received a copy of the GNU General Public License
+#?# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#?#
+#?##################################################################################################
+
+####################################################################################################
+
+#r# ===============
+#r#  Query G-codes
+#r# ===============
+#r#
+#r# For API see
+#r#
+#r# * :mod:`PythonicGcodeMachine.Gcode.Rs274`
+#r# * :mod:`PythonicGcodeMachine.Gcode.Rs274.Config`
+#r# * :mod:`PythonicGcodeMachine.Gcode.Rs274.Machine`
+
+####################################################################################################
+
+from pathlib import Path
+
+from PythonicGcodeMachine.Gcode.Rs274.Machine import GcodeMachine
+
+####################################################################################################
+
+#r# We build a RS-274 G-code Machine
+
+machine = GcodeMachine()
+
+####################################################################################################
+
+#r# We get G-code information
+
+gcode = machine.config.gcodes['G0']
+
+print('Modal group:', gcode.modal_group)
+print('Execution order:', gcode.execution_order)
+#o#
+
+print('\nreStructuredText doc:\n')
+print(gcode.doc)
+#o#
+
+#r# Convert the reStructuredText doc using `pypandoc <https://github.com/bebraw/pypandoc>`_ and
+#r# `Pandoc <https://pandoc.org>`_
+
+try:
+    print('\nMarkdown doc:')
+    print(gcode.convert_doc('md'))
+except (ImportError, RuntimeError):
+    pass
+#o#
+
+try:
+    print('\nHTML doc:')
+    print(gcode.convert_doc('html5'))
+except (ImportError, RuntimeError):
+    pass
+#o#