diff --git a/conservancy_beancount/plugin/core.py b/conservancy_beancount/plugin/core.py
new file mode 100644
index 0000000000000000000000000000000000000000..7ff940eebe6043de1aa47c5d735b73a9ea2b617b
--- /dev/null
+++ b/conservancy_beancount/plugin/core.py
@@ -0,0 +1,54 @@
+"""Base classes for plugin checks"""
+# Copyright © 2020  Brett Smith
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero 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 Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+from . import errors as errormod
+
+class PostingChecker:
+    VALUES_ENUM = {}
+
+    def _meta_get(self, txn, post, key, default=None):
+        try:
+            return post.meta[key]
+        except (KeyError, TypeError):
+            return txn.meta.get(key, default)
+
+    def _meta_set(self, post, key, value):
+        if post.meta is None:
+            post.meta = {}
+        post.meta[key] = value
+
+    def _default_value(self, txn, post):
+        raise errormod.InvalidMetadataError(txn, post, self.METADATA_KEY)
+
+    def check(self, txn, post):
+        errors = []
+        source_value = self._meta_get(txn, post, self.METADATA_KEY)
+        set_value = source_value
+        if source_value is None:
+            try:
+                set_value = self._default_value(txn, post)
+            except errormod._BaseError as error:
+                errors.append(error)
+        else:
+            try:
+                set_value = self.VALUES_ENUM[source_value].value
+            except KeyError:
+                errors.append(errormod.InvalidMetadataError(
+                    txn, post, self.METADATA_KEY, source_value,
+                ))
+        if not errors:
+            self._meta_set(post, self.METADATA_KEY, set_value)
+        return errors