diff --git a/src/Tahoe/SDMF/Internal/Capability.hs b/src/Tahoe/SDMF/Internal/Capability.hs
index 52201b4373030ce0e1f7a8d7cdce1e9cfc08d338..56d6ed1616af9478126c40404844e279d0ee43bd 100644
--- a/src/Tahoe/SDMF/Internal/Capability.hs
+++ b/src/Tahoe/SDMF/Internal/Capability.hs
@@ -6,12 +6,14 @@ import Prelude hiding (Read)
 import qualified Data.ByteString as B
 import Tahoe.SDMF.Internal.Keys (Read, Write)
 
+-- | A read capability for an SDMF object.
 data Reader = Reader
     { readerReadKey :: Read
     , readerVerificationKeyHash :: B.ByteString
     }
     deriving (Show)
 
+-- | A write capability for an SDMF object.
 data Writer = Writer
     { writerWriteKey :: Write
     , writerReader :: Reader