diff --git a/sql/table.sql b/sql/table.sql
index cb724deeaf8843913b9d58dc0ec3102a6745c7e3..a7dcf6b043948c965feee1ced3b3b553d90c9d47 100755
--- a/sql/table.sql
+++ b/sql/table.sql
@@ -40,6 +40,11 @@ CREATE TABLE oligo_feature (
 #
 # Table structure for table 'oligo_probe'
 #
+# Note that the primary key contains both the probe ID and the array ID because
+# it is often possible to get the same probe on different arrays 
+# e.g. (older Affy arrays are often subsets of newer arrays).
+# We give them the same oligo_probe_id so that we only have to store the 
+# features once.
 
 CREATE TABLE oligo_probe (