diff --git a/src/wireviz/wireviz.py b/src/wireviz/wireviz.py
index 84fb1821..57016487 100755
--- a/src/wireviz/wireviz.py
+++ b/src/wireviz/wireviz.py
@@ -67,19 +67,46 @@ def check_designators(what, where): # helper function
 
     autogenerated_ids = {}
     for connection in yaml_data['connections']:
-        # TODO: check that items are of alternating type CONNECTOR/FERRULE/FERRULE_LIST and CABLE/WIRE
-        # TODO: special case: loops!
+        # find first component (potentially nested inside list or dict)
+        first_item = connection[0]
+        if isinstance(first_item, list):
+            first_item = first_item[0]
+        elif isinstance(first_item, dict):
+            first_item = list(first_item.keys())[0]
+        elif isinstance(first_item, str):
+            pass
+
+        # check which section the first item belongs to
+        alternating_sections = ['connectors','cables']
+        for index, section in enumerate(alternating_sections):
+            if first_item in yaml_data[section]:
+                expected_index = index
+                break
+        else:
+            raise Exception('First item not found anywhere.')
+        expected_index = 1 - expected_index  # flip once since it is flipped back at the *beginning* of every loop
 
         # check that all iterable items (lists and dicts) are the same length
+        # and that they are alternating between connectors and cables/bundles, starting with either
         itemcount = None
         for item in connection:
+            expected_index = 1 - expected_index  # make sure items alternate between connectors and cables
+            expected_section = alternating_sections[expected_index]
             if isinstance(item, list):
                 itemcount_new = len(item)
+                for subitem in item:
+                    if not subitem in yaml_data[expected_section]:
+                        raise Exception(f'{subitem} is not in {expected_section}')
             elif isinstance(item, dict):
                 if len(item.keys()) != 1:
-                    raise Exception('Dicts may contain only one item here!')
+                    raise Exception('Dicts may contain only one key here!')
                 itemcount_new = len(expand(list(item.values())[0]))
+                subitem = list(item.keys())[0]
+                if not subitem in yaml_data[expected_section]:
+                    raise Exception(f'{subitem} is not in {expected_section}')
             elif isinstance(item, str):
+                if not item in yaml_data[expected_section]:
+                    raise Exception(f'{item} is not in {expected_section}')
                 continue
             if itemcount is not None and itemcount_new != itemcount:
                 raise Exception('All lists and dict lists must be the same length!')
@@ -124,7 +151,7 @@ def check_designators(what, where): # helper function
             else:
                 raise Exception('Unexpected item in connection list')
 
-        # actually connect things using connection list
+        # actually connect components using connection list
         for i, item in enumerate(connection_list):
             id = item[0][0]  # TODO: make more elegant/robust/pythonic
             if id in harness.cables: