diff --git a/.gitignore b/.gitignore
index fb87c15..f4991b8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 .envrc
 .ghc.environment.*
 .ghcid
+Session.vim
 dist-newstyle/
diff --git a/ki/ki.cabal b/ki/ki.cabal
index 2ed6cd1..9339db8 100644
--- a/ki/ki.cabal
+++ b/ki/ki.cabal
@@ -93,6 +93,7 @@ library
     Ki.Internal.NonblockingSTM
     Ki.Internal.Scope
     Ki.Internal.Thread
+    Ki.Internal.ThreadAffinity
 
 test-suite tests
   import: component
diff --git a/ki/src/Ki/Internal/Thread.hs b/ki/src/Ki/Internal/Thread.hs
index 7b4b5fd..4d82eb0 100644
--- a/ki/src/Ki/Internal/Thread.hs
+++ b/ki/src/Ki/Internal/Thread.hs
@@ -22,8 +22,9 @@ import Control.Exception
     asyncExceptionToException,
   )
 import GHC.Conc (STM)
-import Ki.Internal.ByteCount
+import Ki.Internal.ByteCount (ByteCount)
 import Ki.Internal.IO (forkIO, forkOn, tryEitherSTM)
+import Ki.Internal.ThreadAffinity (ThreadAffinity (..))
 
 -- | A thread.
 --
@@ -65,16 +66,6 @@ makeThread threadId action =
 type Tid =
   Int
 
--- | What, if anything, a thread is bound to.
-data ThreadAffinity
-  = -- | Unbound.
-    Unbound
-  | -- | Bound to a capability.
-    Capability Int
-  | -- | Bound to an OS thread.
-    OsThread
-  deriving stock (Eq, Show)
-
 -- forkIO/forkOn/forkOS, switching on affinity
 forkWithAffinity :: ThreadAffinity -> IO () -> IO ThreadId
 forkWithAffinity = \case
diff --git a/ki/src/Ki/Internal/ThreadAffinity.hs b/ki/src/Ki/Internal/ThreadAffinity.hs
new file mode 100644
index 0000000..fb0e19e
--- /dev/null
+++ b/ki/src/Ki/Internal/ThreadAffinity.hs
@@ -0,0 +1,14 @@
+module Ki.Internal.ThreadAffinity
+  ( ThreadAffinity (..),
+  )
+where
+
+-- | What, if anything, a thread is bound to.
+data ThreadAffinity
+  = -- | Unbound.
+    Unbound
+  | -- | Bound to a capability.
+    Capability Int
+  | -- | Bound to an OS thread.
+    OsThread
+  deriving stock (Eq, Show)