diff --git a/docs/database.md b/docs/database.md
index ce9b08e..5f4047e 100644
--- a/docs/database.md
+++ b/docs/database.md
@@ -11,6 +11,8 @@ but it is easier to be able to refer to the entries without a composite foreign
 
 (NOTE: `entryId` is now inferred from `elementId` within sqlite using a generated column, so saying it is "stored in a separate field" might be a stretch)
 
+In addition, the reading element id's are added with `1000000000` to make them unique from the kanji element id's. This reduces the amount of space needed for indices in some locations, because you can simply filter out each part with `>` or `<`.
+
 We used to generate the `elementId` separately from `orderNum` as a sequential id, but it lead to all values
 shifting whenever the data was updated, leading to very big diffs. Making it be a unique composite of data coming
 from the source data itself means that the values will be stable across updates.