diff --git a/Persister-(.world-etc.)-format.md b/Persister-(.world-etc.)-format.md
index 6d6fff6..e3e4966 100644
--- a/Persister-(.world-etc.)-format.md
+++ b/Persister-(.world-etc.)-format.md
@@ -12,6 +12,7 @@ Worlds generally uses the format described by [Java's DataInput](https://docs.or
 * Arrays, vectors, maybenulls: Explained below 
 * MaybeNull: 1 byte as a null indicator (0x01 for null, 0x00 for non-null), followed by an object (as described below) for non-null
 * Class: A Class ID, then if the Class ID has not been encountered before in this file, a String for the class name. If the Class ID has been encountered before, the class name is omitted and the prior name assumed.
+* Vector: Integer representing number of objects, followed by that number of objects.
 * Array: 
 * "Version for X" (not applicable to Persister version): For the first instance for a given X within this file, this is an integer. Otherwise it is omitted and the prior value assumed. Note that X is usually, but not always, a class. Note that for Persister version 1, "versions" are always 0 and never read from the file.