diff --git a/src/euring/fields.py b/src/euring/fields.py
index 2458586..988e82e 100644
--- a/src/euring/fields.py
+++ b/src/euring/fields.py
@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from types import MappingProxyType
+
 from .codes import (
     LOOKUP_ACCURACY_OF_COORDINATES,
     LOOKUP_ACCURACY_OF_DATE,
@@ -59,7 +61,7 @@
 
 # Schema list (definitions), not per-record data.
 
-EURING_FIELDS = [
+EURING_FIELDS = (
     EuringLookupField(
         name="Ringing Scheme",
         key="ringing_scheme",
@@ -416,20 +418,24 @@
         lookup=lookup_place_code,
     ),
     EuringField(name="More Other Marks", key="more_other_marks", euring_type=TYPE_ALPHABETIC, required=False),
-]
-
-# All keys
-EURING_KEYS = [field.key for field in EURING_FIELDS]
-
-# Map keys to index
-EURING_KEY_INDEX = {key: index for index, key in enumerate(EURING_KEYS)}
+)
 
-# Fields per format (as per the EURING Code Manual)
+# These are the field definitions per format as per the EURING Code Manual
 EURING2020_FIELDS = EURING_FIELDS  # 64 fields
 EURING2000PLUS_FIELDS = EURING_FIELDS[:60]
 EURING2000_FIELDS = EURING_FIELDS[:33]
 
-# Keys per format
-EURING2020_KEYS = [field.key for field in EURING2020_FIELDS]
-EURING2000PLUS_KEYS = [field.key for field in EURING2000PLUS_FIELDS]
-EURING2000_KEYS = [field.key for field in EURING2000_FIELDS]
+# All keys and keys per format
+EURING_KEYS = tuple(field.key for field in EURING_FIELDS)
+EURING2020_KEYS = tuple(field.key for field in EURING2020_FIELDS)
+EURING2000PLUS_KEYS = tuple(field.key for field in EURING2000PLUS_FIELDS)
+EURING2000_KEYS = tuple(field.key for field in EURING2000_FIELDS)
+
+# Mapping keys to indexes and names
+EURING_KEY_INDEX = MappingProxyType({key: index for index, key in enumerate(EURING_KEYS)})
+EURING_KEY_NAME = MappingProxyType({field.key: field.name for field in EURING_FIELDS})
+
+# Helpers
+EURING2020_ONLY_KEYS = tuple(set(EURING2020_KEYS).difference(EURING2000PLUS_KEYS))
+NON_EURING2000_KEYS = tuple(set(EURING2000PLUS_KEYS + EURING2020_ONLY_KEYS).difference(EURING2000_KEYS))
+NON_EURING2000PLUS_KEYS = tuple(key for key in EURING_KEYS if key not in EURING2000PLUS_KEYS)
diff --git a/src/euring/rules.py b/src/euring/rules.py
index 9489dcb..845ba8c 100644
--- a/src/euring/rules.py
+++ b/src/euring/rules.py
@@ -2,17 +2,15 @@
 
 from __future__ import annotations
 
-from .fields import EURING2000_FIELDS, EURING2000PLUS_FIELDS, EURING2020_FIELDS, EURING_FIELDS
+from .fields import (
+    EURING2020_ONLY_KEYS,
+    EURING_FIELDS,
+    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}
 
-EURING2000_KEYS = tuple(field["key"] for field in EURING2000_FIELDS)
-EURING2000PLUS_KEYS = tuple(field["key"] for field in EURING2000PLUS_FIELDS)
-EURING2020_KEYS = tuple(field["key"] for field in EURING2020_FIELDS)
-EURING2020_ONLY_KEYS = tuple(set(EURING2020_KEYS).difference(EURING2000PLUS_KEYS))
-NON_EURING2000_KEYS = tuple(set(EURING2000PLUS_KEYS + EURING2020_ONLY_KEYS).difference(EURING2000_KEYS))
-
 
 def field_name_for_key(key: str) -> str:
     """Return the field name for a key, falling back to the key."""
diff --git a/tests/fixtures/__init__.py b/tests/fixtures/__init__.py
index b8d4196..db8adad 100644
--- a/tests/fixtures/__init__.py
+++ b/tests/fixtures/__init__.py
@@ -7,7 +7,7 @@
 )
 from euring.formats import FORMAT_EURING2000, FORMAT_EURING2000PLUS, FORMAT_EURING2020
 
-DEFAULT_TEST_VALUES = {
+EURING_TEST_DATA = {
     # Default test data for a EURING record in key-value format
     "ringing_scheme": "GBB",
     "primary_identification_method": "A0",
@@ -33,7 +33,7 @@
     "accuracy_of_date": "0",
     "time": "0000",
     "place_code": "AB00",
-    "geographical_coordinates": "+0000000+0000000",
+    "geographical_coordinates": "+000000+0000000",
     "accuracy_of_coordinates": "1",
     "condition": "9",
     "circumstances": "99",
@@ -77,24 +77,31 @@
     "more_other_marks": "",
 }
 
+EURING2000_TEST_DATA = {key: value for key, value in EURING_TEST_DATA.items() if key in EURING2000_KEYS}
+EURING2000PLUS_TEST_DATA = {key: value for key, value in EURING_TEST_DATA.items() if key in EURING2000PLUS_KEYS}
+EURING2020_TEST_DATA = {key: value for key, value in EURING_TEST_DATA.items() if key in EURING2020_KEYS}
+
 
 def _make_euring_record(data: dict, format: str) -> str:
+    record = dict()
     if format == FORMAT_EURING2000:
         keys = EURING2000_KEYS
+        record.update(EURING2000_TEST_DATA)
         separator = ""
     elif format == FORMAT_EURING2000PLUS:
         keys = EURING2000PLUS_KEYS
+        record.update(EURING2000PLUS_TEST_DATA)
         separator = "|"
     elif format == FORMAT_EURING2020:
         keys = EURING2020_KEYS
+        record.update(EURING2020_TEST_DATA)
         separator = "|"
     else:
         raise ValueError(f"Unknown format: {format}")
-    record_dict = {key: value for key, value in DEFAULT_TEST_VALUES.items() if key in keys}
     for key, value in data.items():
         assert key in keys, f"Invalid key: {key}"
-        record_dict[key] = value
-    return separator.join(record_dict.values())
+        record[key] = value
+    return separator.join(record.get(key, "") for key in keys)
 
 
 def _make_euring2000_record(**kwargs) -> str:
diff --git a/tests/test_rules.py b/tests/test_rules.py
index 3c74f6c..089a0a7 100644
--- a/tests/test_rules.py
+++ b/tests/test_rules.py
@@ -1,11 +1,13 @@
 """Tests for record_rules helpers."""
 
-from euring.rules import (
+from euring.fields import (
     EURING2000_KEYS,
     EURING2000PLUS_KEYS,
     EURING2020_KEYS,
     EURING2020_ONLY_KEYS,
     NON_EURING2000_KEYS,
+)
+from euring.rules import (
     matches_euring2000,
     record_rule_errors,
     requires_euring2000plus,