diff --git a/modules/styling-transform/spec/utils/handleCreateStencil.spec.ts b/modules/styling-transform/spec/utils/handleCreateStencil.spec.ts
index 3d71e360f2..d8e5c97f12 100644
--- a/modules/styling-transform/spec/utils/handleCreateStencil.spec.ts
+++ b/modules/styling-transform/spec/utils/handleCreateStencil.spec.ts
@@ -485,6 +485,56 @@ describe('handleCreateStencil', () => {
       );
     });
 
+    it('should handle modifiers with fallthrough keys', () => {
+      const program = createProgramFromSource(`
+        import {createStencil, px2rem} from '@workday/canvas-kit-styling';
+
+        const buttonStencil = createStencil({
+          vars: {
+            padding: ''
+          },
+          base: {},
+          modifiers: {
+            padding: {
+              large: {
+                padding: 40,
+              },
+              small: {
+                padding: 20,
+              }
+              _: ({ padding }) => ({
+                padding
+              })
+            }
+          }
+        })
+      `);
+
+      const styles = {};
+      const names = {};
+      const extractedNames = {};
+
+      const result = transform(
+        program,
+        'test.ts',
+        withDefaultContext(program.getTypeChecker(), {styles, names, extractedNames})
+      );
+
+      expect(result).toMatch(/base: { name: "[0-9a-z]+", styles: "box-sizing:border-box;" }/);
+      expect(result).toMatch(/large: { name: "[0-9a-z]+", styles: "padding:40px;" }/);
+      expect(result).toMatch(
+        /_: { name: "[0-9a-z]+", styles: "padding:var\(--padding-button-[a-z0-9]+\);" }/
+      );
+
+      expect(styles['test.css']).toContainEqual(compileCSS('.css-button{box-sizing:border-box;}'));
+      expect(styles['test.css']).toContainEqual(
+        compileCSS('.css-button.padding-large{padding: 40px;}')
+      );
+      expect(styles['test.css']).toContainEqual(
+        compileCSS('.css-button.padding{padding: var(--css-button-padding)}')
+      );
+    });
+
     it('should add to extracted styles', () => {
       const program = createProgramFromSource(`
         import {createStencil} from '@workday/canvas-kit-styling';
diff --git a/modules/styling/lib/cs.ts b/modules/styling/lib/cs.ts
index d5347b8c4e..b2786f23c5 100644
--- a/modules/styling/lib/cs.ts
+++ b/modules/styling/lib/cs.ts
@@ -415,8 +415,10 @@ export function createModifiers<M extends ModifierConfig>(input: M): ModifierRet
   const modifierFn = (modifiers: Partial<ModifierValues<M>>) => {
     return combineClassNames(
       Object.keys(input)
-        .filter(key => (input as any)[key][modifiers[key]])
-        .map(key => (input as any)[key][modifiers[key]])
+        .map(
+          key => (input as any)[key][modifiers[key]] || (modifiers[key] && (input as any)[key]._)
+        )
+        .filter(input => input) // only return defined class names
     );
   };
 
diff --git a/modules/styling/spec/cs.spec.tsx b/modules/styling/spec/cs.spec.tsx
index c4b139863d..f893def599 100644
--- a/modules/styling/spec/cs.spec.tsx
+++ b/modules/styling/spec/cs.spec.tsx
@@ -348,6 +348,29 @@ describe('cs', () => {
       const myModifiers = myModifiersFactory();
       expectTypeOf(myModifiers.size).toMatchTypeOf<{large: CS; small: CS}>();
     });
+
+    describe('with a "_" modifier key', () => {
+      const myModifiersFactory = () =>
+        createModifiers({
+          size: {
+            large: createStyles({fontSize: '1.5rem'}),
+            small: createStyles({fontSize: '0.8rem'}),
+            _: createStyles({fontSize: 'var(--size)'}),
+          },
+        });
+
+      it('should not return any classes when "size" is not provided', () => {
+        const myModifiers = myModifiersFactory();
+
+        expect(myModifiers({})).toEqual('');
+      });
+
+      it('should return the CSS class of the "_" key when no other modifier matches and a matching modifier key is provided', () => {
+        const myModifiers = myModifiersFactory();
+
+        expect(myModifiers({size: 'foo' as any})).toEqual(myModifiers.size._);
+      });
+    });
   });
 
   describe('createCompoundModifiers', () => {