diff --git a/sources/contactusage.h b/sources/contactusage.h
index 5b29176ca..406a6223a 100644
--- a/sources/contactusage.h
+++ b/sources/contactusage.h
@@ -29,6 +29,12 @@
 	rules can be unit tested on their own. MasterElement::contactUsage()
 	is the thin wrapper that feeds it the linked elements.
 
+	This counts contacts, which is what tells you how many contacts an
+	auxiliary block must provide. It is deliberately not the count that
+	MasterElement::isFull() uses: a master's max_slaves is a number of
+	slots, and a slave fills exactly one slot however many contacts it
+	carries.
+
 	Two rules are easy to get wrong, and both live here so that every
 	caller gets them right:
 	 - a slave stands for as many contacts as its "number" kind
diff --git a/sources/qetgraphicsitem/masterelement.cpp b/sources/qetgraphicsitem/masterelement.cpp
index 1ccebfd3a..599b8c90d 100644
--- a/sources/qetgraphicsitem/masterelement.cpp
+++ b/sources/qetgraphicsitem/masterelement.cpp
@@ -282,8 +282,11 @@ bool MasterElement::isFull() const
 		return false;
 	}
 
-	// Return true if the contacts already used reached or exceeded the limit
-	return contactUsage().total() >= max_slaves;
+		// max_slaves is a number of slots, not of contacts: it sizes the
+		// element's contact group table, and a slave occupies exactly one
+		// group however many contacts that group stands for. So the slots
+		// in use are the linked elements, not the contacts they carry.
+	return connected_elements.size() >= max_slaves;
 }
 
 /**