diff --git a/hwtypes/adt_meta.py b/hwtypes/adt_meta.py
index cb052e7..b5caf38 100644
--- a/hwtypes/adt_meta.py
+++ b/hwtypes/adt_meta.py
@@ -1,6 +1,6 @@
+from abc import ABCMeta, abstractmethod
 import itertools as it
 import typing as tp
-from abc import ABCMeta, abstractmethod
 import weakref
 
 from types import MappingProxyType
@@ -53,15 +53,27 @@ RESERVED_SUNDERS = frozenset({
     '_fields_',
     '_field_table_',
     '_unbound_base_',
+    '_syntax_',
 })
 
 RESERVED_ATTRS = frozenset(RESERVED_NAMES | RESERVED_SUNDERS)
 
+class Syntax(type):
+    def __subclasscheck__(cls, sub):
+        return super().__subclasscheck__(getattr(sub, '_syntax_', type(None)))
+
+    def __instancecheck__(cls, instance):
+        return cls.__subclasscheck__(type(instance))
+
+
+class AttrSyntax(metaclass=Syntax): pass
+class GetitemSyntax(metaclass=Syntax): pass
 
 # Can't have abstract metaclass https://bugs.python.org/issue36881
-class GetitemSyntax: #(metaclass=ABCMeta):
+class _GetitemSyntax(type): #(metaclass=ABCMeta):
     # Should probaly do more than it currently does but it gets the job done
     ORDERED = False
+    _syntax_ = GetitemSyntax
 
     def __getitem__(cls, idx):
         if not isinstance(idx, tp.Iterable):
@@ -78,10 +90,12 @@ class GetitemSyntax: #(metaclass=ABCMeta):
     def _from_idx(cls, idx):
         pass
 
-class AttrSyntax(type): #, metaclass=ABCMeta):
+
+class _AttrSyntax(type): #, metaclass=ABCMeta):
     # Tells AttrSyntax which attrs are fields
     FIELDS_T = type
     ORDERED =  False
+    _syntax_ = AttrSyntax
 
     def __new__(mcs, name, bases, namespace, cache=False, **kwargs):
         fields = {}
@@ -160,7 +174,8 @@ class AttrSyntax(type): #, metaclass=ABCMeta):
     def _from_fields(mcs, fields, name, bases, ns, **kwargs):
         pass
 
-class BoundMeta(GetitemSyntax, type): #, metaclass=ABCMeta):
+
+class BoundMeta(_GetitemSyntax): #, metaclass=ABCMeta):
     # for legacy reasons
     ORDERED = True
 
@@ -284,6 +299,7 @@ class BoundMeta(GetitemSyntax, type): #, metaclass=ABCMeta):
     def is_cached(cls):
         return getattr(cls, '_cached_', False)
 
+
 class TupleMeta(BoundMeta):
     ORDERED = True
 
@@ -309,7 +325,7 @@ class TupleMeta(BoundMeta):
         return MappingProxyType({idx : field for idx, field in enumerate(cls.fields)})
 
 
-class ProductMeta(AttrSyntax, TupleMeta):
+class ProductMeta(_AttrSyntax, TupleMeta):
     FIELDS_T = type
     ORDERED = True
 
@@ -437,7 +453,7 @@ class SumMeta(BoundMeta):
         return MappingProxyType({field : field for field in cls.fields})
 
 
-class TaggedUnionMeta(AttrSyntax, SumMeta):
+class TaggedUnionMeta(_AttrSyntax, SumMeta):
     FIELDS_T = type
     ORDERED = False
 
@@ -495,12 +511,11 @@ class TaggedUnionMeta(AttrSyntax, SumMeta):
                 yield cls(**{tag: field()})
 
 
-class EnumMeta(AttrSyntax, BoundMeta):
+class EnumMeta(_AttrSyntax, BoundMeta):
     class Auto:
         def __repr__(self):
             return 'Auto()'
 
-
     FIELDS_T = int, Auto
     ORDERED = False
 
@@ -514,23 +529,23 @@ class EnumMeta(AttrSyntax, BoundMeta):
                 else:
                     raise TypeError('Can only inherit from one enum type')
 
-        t = super().__new__(mcs, name, bases, ns, **kwargs)
+        cls = super().__new__(mcs, name, bases, ns, **kwargs)
 
         name_table = dict()
         for name, value in fields.items():
-            elem = t.__new__(t)
-            t.__init__(elem, value)
+            elem = cls.__new__(cls)
+            cls.__init__(elem, value)
             setattr(elem, '_name_', name)
             name_table[name] = elem
-            setattr(t, name, elem)
+            setattr(cls, name, elem)
 
-        t._fields_ = tuple(name_table.values())
-        t._field_table_ = name_table
+        cls._fields_ = tuple(name_table.values())
+        cls._field_table_ = name_table
 
         if enum_base is not None:
-            t._unbound_base_ = enum_base
+            cls._unbound_base_ = enum_base
 
-        return t
+        return cls
 
     def __call__(cls, elem):
         if not isinstance(elem, cls):

diff --git a/tests/test_adt.py b/tests/test_adt.py
index 03c0a8b..0472b15 100644
--- a/tests/test_adt.py
+++ b/tests/test_adt.py
@@ -1,8 +1,10 @@
 import pytest
 from hwtypes.adt import Product, Sum, Enum, Tuple, TaggedUnion
-from hwtypes.adt_meta import RESERVED_ATTRS, ReservedNameError
+from hwtypes.adt_meta import RESERVED_ATTRS, ReservedNameError, AttrSyntax, GetitemSyntax
 from hwtypes.modifiers import new
 from hwtypes.adt_util import rebind_bitvector
+from hwtypes import BitVector, AbstractBitVector, Bit, AbstractBit
+
 
 class En1(Enum):
     a = 0
@@ -386,3 +388,18 @@ def test_unbound_t(t, base):
 def test_deprecated(val):
     with pytest.warns(DeprecationWarning):
         val.value
+
+def test_adt_syntax():
+    # En1, Pr, Su, Tu, Ta
+    for T in (En1, Pr, Ta):
+        assert isinstance(T, AttrSyntax)
+        assert not isinstance(T, GetitemSyntax)
+
+    for T in (Su, Tu):
+        assert not isinstance(T, AttrSyntax)
+        assert isinstance(T, GetitemSyntax)
+
+    for T in (str, Bit, BitVector[4], AbstractBit, AbstractBitVector[4], int):
+        assert not isinstance(T, AttrSyntax)
+        assert not isinstance(T, GetitemSyntax)
+

Syntax's are not working correctly with isinstance()
See the failing test test_adt_syntax in tests/test_adt.py in branch 'adt_meta'
tests/test_adt.py::test_repr[Sum[En1, tests/test_adt.py::test_reserved[_syntax_-T_field0] tests/test_adt.py::test_reserved[_syntax_-T_field1] tests/test_adt.py::test_reserved[_syntax_-T_field2] tests/test_adt.py::test_unbound_t[Sum[En1, tests/test_adt.py::test_adt_syntax
tests/test_adt.py::test_enum tests/test_adt.py::test_tuple tests/test_adt.py::test_product tests/test_adt.py::test_product_from_fields tests/test_adt.py::test_product_caching tests/test_adt.py::test_sum tests/test_adt.py::test_tagged_union tests/test_adt.py::test_tagged_union_from_fields tests/test_adt.py::test_tagged_union_caching tests/test_adt.py::test_new tests/test_adt.py::test_repr[En1] tests/test_adt.py::test_repr[Tuple[En1, tests/test_adt.py::test_repr[Pr] tests/test_adt.py::test_repr[Ta] tests/test_adt.py::test_reserved[enumerate-T_field0] tests/test_adt.py::test_reserved[enumerate-T_field1] tests/test_adt.py::test_reserved[enumerate-T_field2] tests/test_adt.py::test_reserved[fields-T_field0] tests/test_adt.py::test_reserved[fields-T_field1] tests/test_adt.py::test_reserved[fields-T_field2] tests/test_adt.py::test_reserved[_fields_-T_field0] tests/test_adt.py::test_reserved[_fields_-T_field1] tests/test_adt.py::test_reserved[_fields_-T_field2] tests/test_adt.py::test_reserved[_value_-T_field0] tests/test_adt.py::test_reserved[_value_-T_field1] tests/test_adt.py::test_reserved[_value_-T_field2] tests/test_adt.py::test_reserved[_unbound_base_-T_field0] tests/test_adt.py::test_reserved[_unbound_base_-T_field1] tests/test_adt.py::test_reserved[_unbound_base_-T_field2] tests/test_adt.py::test_reserved[is_bound-T_field0] tests/test_adt.py::test_reserved[is_bound-T_field1] tests/test_adt.py::test_reserved[is_bound-T_field2] tests/test_adt.py::test_reserved[_tag_-T_field0] tests/test_adt.py::test_reserved[_tag_-T_field1] tests/test_adt.py::test_reserved[_tag_-T_field2] tests/test_adt.py::test_reserved[is_cached-T_field0] tests/test_adt.py::test_reserved[is_cached-T_field1] tests/test_adt.py::test_reserved[is_cached-T_field2] tests/test_adt.py::test_reserved[_field_table_-T_field0] tests/test_adt.py::test_reserved[_field_table_-T_field1] tests/test_adt.py::test_reserved[_field_table_-T_field2] tests/test_adt.py::test_reserved[field_dict-T_field0] tests/test_adt.py::test_reserved[field_dict-T_field1] tests/test_adt.py::test_reserved[field_dict-T_field2] tests/test_adt.py::test_reserved[_cached_-T_field0] tests/test_adt.py::test_reserved[_cached_-T_field1] tests/test_adt.py::test_reserved[_cached_-T_field2] tests/test_adt.py::test_reserved[value_dict-T_field0] tests/test_adt.py::test_reserved[value_dict-T_field1] tests/test_adt.py::test_reserved[value_dict-T_field2] tests/test_adt.py::test_unbound_t[En1-Enum] tests/test_adt.py::test_unbound_t[Pr-Product] tests/test_adt.py::test_unbound_t[Tuple[En1, tests/test_adt.py::test_unbound_t[Ta-TaggedUnion] tests/test_adt.py::test_deprecated[val0] tests/test_adt.py::test_deprecated[val1] tests/test_adt.py::test_deprecated[val2] tests/test_adt.py::test_deprecated[val3] tests/test_adt.py::test_deprecated[val4]