diff --git a/.flake8 b/.flake8
new file mode 100644
index 0000000..1d1c7f5
--- /dev/null
+++ b/.flake8
@@ -0,0 +1,8 @@
+# vim: ft=cfg
+[flake8]
+max-complexity = 7
+docstring-convention=google
+
+# Required to be compatible with black
+extend-ignore = E203,W503
+inline-quotes = double
diff --git a/app/exceptions.py b/app/exceptions.py
new file mode 100644
index 0000000..b82bad8
--- /dev/null
+++ b/app/exceptions.py
@@ -0,0 +1,55 @@
+"""Common exceptions raised by the program."""
+from typing import Union, List
+
+
+class InvalidKeyError(Exception):
+    """Thrown when a config file contains an invalid key."""
+
+    def __init__(self, keys: Union[str, List[str]]):
+        """Create a new InvalidKeyError object with the given key.
+
+        Args:
+            keys: the invalid key(s)
+        """
+        if type(keys) == str:
+            keys = [keys]
+
+        self.message = "Invalid key(s): {}".format(", ".join(keys))
+
+        super().__init__()
+
+
+class MissingKeyError(Exception):
+    """Thrown when a required key is missing from a config."""
+
+    def __init__(self, keys: Union[str, List[str]]):
+        """Create a new MissingKeyError object with the given key.
+
+        Args:
+            keys: the invalid key(s)
+        """
+        if type(keys) == str:
+            keys = [keys]
+
+        self.message = "Missing key(s): {}".format(", ".join(keys))
+
+        super().__init__()
+
+
+class InvalidValueError(Exception):
+    """Thrown when a key contains an invalid value."""
+
+    def __init__(self, key: str, expected: str, actual: str):
+        """Create a new InvalidValueError given the arguments.
+
+        Args:
+            key: the key containing the invalid value
+            expected: name of the expected type
+            actual: name of the actual type
+        """
+        self.message = (
+            f"Invalid value for key {key}: expected {expected}, "
+            f"got {actual}"
+        )
+
+        super().__init__()
diff --git a/app/skeleton.py b/app/skeleton.py
new file mode 100644
index 0000000..c12bd45
--- /dev/null
+++ b/app/skeleton.py
@@ -0,0 +1,93 @@
+"""Handles merging with the skeleton config."""
+from typing import Dict
+from .exceptions import InvalidKeyError, MissingKeyError
+
+
+def merge(*dicts: [Dict]) -> Dict:
+    """Merge multiple dicts into one.
+
+    It reads the dicts from left to right, always preferring the "right"
+    dictionary's values. Therefore, the dictionaries should be sorted from
+    least important to most important (e.g. a default values skeleton should be
+    to the left of a dict of selected values).
+
+    Args:
+        dicts: the dictionaries to merge
+
+    Returns:
+        a new dictionary representing the merged dictionaries
+
+    Todo:
+        * Make sure an infinite loop is not possible
+    """
+    # Base cases
+    if len(dicts) == 0:
+        return {}
+
+    if len(dicts) == 1:
+        return dicts[0]
+
+    # We merge the first two dicts
+    d1, d2 = dicts[0], dicts[1]
+
+    output = d1.copy()
+
+    for key, value in d2.items():
+        if type(value) == dict:
+            # Merge the two sub-dictionaries
+            output[key] = (
+                merge(output[key], value)
+                if type(output.get(key)) == dict
+                else value
+            )
+
+        else:
+            output[key] = value
+
+    return merge(output, *dicts[2:])
+
+
+def merge_with_skeleton(data: Dict, skel: Dict) -> Dict:
+    """Merge a dictionary with a skeleton containing default values.
+
+    The skeleton not only defines what the default values are, but also
+    enforces a certain shape. This allows us to define a config file using a
+    dictionary and parse it.
+
+    Args:
+        data: dictionary containing the selected config values
+        skel: dictionary containing the skeleton (aka the def)
+
+    Returns:
+        a new dictionary representing the two merged dictionaries
+
+    Todo:
+        * Check if an infinite loop is possible
+        * Split info less complex functions
+    """
+    # First, check for illegal keys
+    invalid_keys = list(filter(lambda k: k not in skel, data))
+
+    if invalid_keys:
+        raise InvalidKeyError(invalid_keys)
+
+    # Then, check the default values
+    for key, value in skel.items():
+        if key not in data:
+            # Raise error if there's not default value
+            if value is None:
+                raise MissingKeyError(key)
+
+            # Replace with default value
+            data[key] = value
+
+        # Error if value is not same type as default value
+        elif type(data[key]) != type(value) and value is not None:
+            # TODO make this error message more verbose
+            raise TypeError("Invalid value type")
+
+        # Recurse into dicts
+        elif type(value) == dict:
+            data[key] = merge_with_skeleton(data[key], value)
+
+    return data
diff --git a/pyproject.toml b/pyproject.toml
index 9787c3b..46e8364 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,3 +1,6 @@
 [build-system]
 requires = ["setuptools", "wheel"]
 build-backend = "setuptools.build_meta"
+
+[tool.black]
+line-length = 79
diff --git a/tests/test_dict_merge.py b/tests/test_dict_merge.py
new file mode 100644
index 0000000..2da81bf
--- /dev/null
+++ b/tests/test_dict_merge.py
@@ -0,0 +1,69 @@
+"""Tests for the skeleton module."""
+from app.skeleton import merge
+
+
+def test_merge_empty():
+    """Test correct response for an empty merge."""
+    assert merge() == {}
+
+
+def test_merge_single():
+    """Test merge command with a single input."""
+    assert merge({}) == {}
+
+    dic = {"test": "value", "test2": "value2"}
+
+    assert merge(dic) == dic
+
+
+def test_merge_double_no_overlap():
+    """Test merge command with two non-overlapping inputs."""
+    d1 = {"test": "value", "test2": "value2"}
+    d2 = {"test3": "value3"}
+    d_res = {"test": "value", "test2": "value2", "test3": "value3"}
+
+    assert merge(d1, d2) == d_res
+
+
+def test_merge_double_overlap():
+    """Test merge command with two overlapping inputs."""
+    d1 = {"test": "value", "test2": "value2"}
+    d2 = {"test2": "value3"}
+    d_res = {"test": "value", "test2": "value3"}
+
+    assert merge(d1, d2) == d_res
+
+
+def test_merge_triple_no_overlap():
+    """Test merge command with three non-overlapping inputs.
+
+    This test tells us that the recursion works.
+    """
+    d1 = {"test": "value", "test2": "value2"}
+    d2 = {"test3": "value3"}
+    d3 = {"test4": "value4"}
+    d_res = {
+        "test": "value",
+        "test2": "value2",
+        "test3": "value3",
+        "test4": "value4",
+    }
+
+    assert merge(d1, d2, d3) == d_res
+
+
+def test_merge_triple_overlap():
+    """Test merge command with three overlapping inputs.
+
+    This test tells us that the recursion works.
+    """
+    d1 = {"test": "value", "test2": "value2"}
+    d2 = {"test3": "value3"}
+    d3 = {"test2": "value4"}
+    d_res = {
+        "test": "value",
+        "test2": "value4",
+        "test3": "value3",
+    }
+
+    assert merge(d1, d2, d3) == d_res
diff --git a/tests/test_skeleton.py b/tests/test_skeleton.py
new file mode 100644
index 0000000..9ade87e
--- /dev/null
+++ b/tests/test_skeleton.py
@@ -0,0 +1,70 @@
+"""Tests wether the skeleton merge works."""
+from app.skeleton import merge_with_skeleton
+from app.exceptions import InvalidKeyError, MissingKeyError
+import pytest
+
+
+def test_single_invalid_key():
+    """Tests wether an InvalidKeyError is correctly thrown for a single key."""
+    data = {
+        "test": 1,
+        "test2": "test"
+    }
+    skel = {
+        "test": None,
+    }
+
+    with pytest.raises(InvalidKeyError) as e_info:
+        merge_with_skeleton(data, skel)
+
+    assert e_info.value.message == "Invalid key(s): test2"
+
+
+def test_multiple_invalid_keys():
+    """Tests wether an InvalidKeyError is thrown for multiple keys."""
+    data = {
+        "test": 1,
+        "test2": "test",
+        "test3": "test",
+    }
+    skel = {
+        "test": None,
+    }
+
+    with pytest.raises(InvalidKeyError) as e_info:
+        merge_with_skeleton(data, skel)
+
+    assert e_info.value.message == "Invalid key(s): test2, test3"
+
+
+def test_single_missing_key():
+    """Tests wether a MissingKeyError is correctly thrown for a single key."""
+    data = {
+        "test": 1,
+    }
+    skel = {
+        "test": None,
+        "test2": None,
+    }
+
+    with pytest.raises(MissingKeyError) as e_info:
+        merge_with_skeleton(data, skel)
+
+    assert e_info.value.message == "Missing key(s): test2"
+
+
+def test_multiple_missing_keys():
+    """Tests wether a MissingKeyError is correctly thrown for multiple keys."""
+    data = {
+        "test": 1,
+    }
+    skel = {
+        "test": None,
+        "test2": None,
+        "test3": None,
+    }
+
+    with pytest.raises(MissingKeyError) as e_info:
+        merge_with_skeleton(data, skel)
+
+    assert e_info.value.message == "Missing key(s): test2, test3"