diff --git a/extra/disjoint-set/authors.txt b/extra/disjoint-sets/authors.txt
similarity index 100%
rename from extra/disjoint-set/authors.txt
rename to extra/disjoint-sets/authors.txt
diff --git a/extra/disjoint-set/disjoint-set.factor b/extra/disjoint-sets/disjoint-sets.factor
similarity index 93%
rename from extra/disjoint-set/disjoint-set.factor
rename to extra/disjoint-sets/disjoint-sets.factor
index 6f3b1e63e8..f56ee7ccfe 100644
--- a/extra/disjoint-set/disjoint-set.factor
+++ b/extra/disjoint-sets/disjoint-sets.factor
@@ -1,6 +1,8 @@
+! Copyright (C) 2008 Eric Mertens.
+! See http://factorcode.org/license.txt for BSD license.
 USING: accessors arrays hints kernel locals math sequences ;
 
-IN: disjoint-set
+IN: disjoint-sets
 
 <PRIVATE
 
diff --git a/extra/disjoint-set/summary.txt b/extra/disjoint-sets/summary.txt
similarity index 100%
rename from extra/disjoint-set/summary.txt
rename to extra/disjoint-sets/summary.txt
diff --git a/extra/disjoint-set/tags.txt b/extra/disjoint-sets/tags.txt
similarity index 100%
rename from extra/disjoint-set/tags.txt
rename to extra/disjoint-sets/tags.txt
diff --git a/extra/project-euler/186/186.factor b/extra/project-euler/186/186.factor
index acec27c51f..ce2b17a610 100644
--- a/extra/project-euler/186/186.factor
+++ b/extra/project-euler/186/186.factor
@@ -1,4 +1,4 @@
-USING: circular disjoint-set kernel math math.ranges
+USING: circular disjoint-sets kernel math math.ranges
        sequences sequences.lib ;
 IN: project-euler.186