diff --git a/src/euring/rules.py b/src/euring/rules.py
index 845ba8c..5d87405 100644
--- a/src/euring/rules.py
+++ b/src/euring/rules.py
@@ -2,29 +2,23 @@
 
 from __future__ import annotations
 
-from .fields import (
-    EURING2020_ONLY_KEYS,
-    EURING_FIELDS,
-    NON_EURING2000_KEYS,
-)
+from .fields import EURING2020_ONLY_KEYS, EURING_KEY_NAME, NON_EURING2000_KEYS
 from .formats import FORMAT_EURING2000, FORMAT_EURING2000PLUS, FORMAT_EURING2020
 
-_FIELD_NAME_BY_KEY = {field["key"]: field["name"] for field in EURING_FIELDS}
-
 
 def field_name_for_key(key: str) -> str:
     """Return the field name for a key, falling back to the key."""
-    return _FIELD_NAME_BY_KEY.get(key, key)
+    return EURING_KEY_NAME.get(key, key)
 
 
-def accuracy_is_alpha(values_by_key: dict[str, str]) -> bool:
+def accuracy_of_coordinates_is_alpha(values_by_key: dict[str, str]) -> bool:
     """Return True when accuracy_of_coordinates is alphabetic."""
-    accuracy = values_by_key.get("accuracy_of_coordinates", "")
-    return bool(accuracy) and accuracy.isalpha()
+    accuracy_of_coordinates = values_by_key.get("accuracy_of_coordinates", "")
+    return bool(accuracy_of_coordinates) and accuracy_of_coordinates.isalpha()
 
 
 def matches_euring2000(values_by_key: dict[str, str]) -> bool:
-    """Return True when values fit EURING2000."""
+    """Return True when values are present only for fields in EURING2000 format."""
     for key in NON_EURING2000_KEYS:
         if values_by_key.get(key):
             return False
@@ -32,13 +26,13 @@ def matches_euring2000(values_by_key: dict[str, str]) -> bool:
 
 
 def requires_euring2000plus(values_by_key: dict[str, str]) -> bool:
-    """Return True when values require EURING2000+."""
+    """Return True when values are present for fields that are not in EURING2000 format."""
     return not matches_euring2000(values_by_key)
 
 
 def requires_euring2020(values_by_key: dict[str, str]) -> bool:
     """Return True when values require EURING2020."""
-    if accuracy_is_alpha(values_by_key):
+    if accuracy_of_coordinates_is_alpha(values_by_key):
         return True
     for key in EURING2020_ONLY_KEYS:
         if values_by_key.get(key):
@@ -58,22 +52,22 @@ def _error(key, message):
         }
 
     if format == FORMAT_EURING2020:
-        geo_value = values_by_key.get("geographical_coordinates", "") or ""
-        lat_value = values_by_key.get("latitude", "") or ""
-        lng_value = values_by_key.get("longitude", "") or ""
-        if lat_value or lng_value:
-            if geo_value and geo_value != "." * 15:
+        geographical_coordinates = values_by_key.get("geographical_coordinates", "") or ""
+        latitude = values_by_key.get("latitude", "") or ""
+        longitude = values_by_key.get("longitude", "") or ""
+        if latitude or longitude:
+            if geographical_coordinates and geographical_coordinates != "." * 15:
                 errors.append(
                     _error(
                         key="geographical_coordinates",
                         message="When Latitude/Longitude are provided, Geographical Co-ordinates must be 15 dots.",
                     )
                 )
-        if lat_value and not lng_value:
+        if latitude and not longitude:
             errors.append(
                 _error(key="longitude", message="Longitude is required when Latitude is provided."),
             )
-        if lng_value and not lat_value:
+        if longitude and not latitude:
             errors.append(
                 _error(
                     key="latitude",
@@ -81,7 +75,7 @@ def _error(key, message):
                 )
             )
     else:
-        if accuracy_is_alpha(values_by_key):
+        if accuracy_of_coordinates_is_alpha(values_by_key):
             errors.append(
                 _error(
                     key="accuracy_of_coordinates",