diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 8595280..199859b 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -24,7 +24,7 @@ If you wish to contribute bugfixes or code to the BOSL2 project, the standard wa
 	    3. Click the Clone button.
 	    4. When it asks "How are you planning to use this fork?", click on the button "To contribute to the parent project."
 	
-1. Before you edit files, always syncronize with the upstream repository:
+1. Before you edit files, always synchronize with the upstream repository:
     - If using the command-line:
         ```
         git pull upstream
diff --git a/paths.scad b/paths.scad
index bdb05fb..fe14a50 100644
--- a/paths.scad
+++ b/paths.scad
@@ -747,16 +747,20 @@ function assemble_path_fragments(fragments, eps=EPSILON, _finished=[]) =
 // Arguments:
 //   r = Radius of the base circle. Default: 40
 //   d = Diameter of the base circle.
-//   sines = array of [amplitude, frequency] pairs, where the frequency is the number of times the cycle repeats around the circle.
+//   sines = array of [amplitude, frequency] pairs or [amplitude, frequency, phase] triples, where the frequency is the number of times the cycle repeats around the circle.
 // Example(2D):
 //   modulated_circle(r=40, sines=[[3, 11], [1, 31]], $fn=6);
-module modulated_circle(r, sines=[10], d)
+module modulated_circle(r, sines=[[1,1]], d)
 {
     r = get_radius(r=r, d=d, dflt=40);
-    freqs = len(sines)>0? [for (i=sines) i[1]] : [5];
+    assert(is_list(sines)
+        && all([for(s=sines) is_vector(s,2) || is_vector(s,3)]),
+        "sines must be given as a list of pairs or triples");
+    sines_ = [for(s=sines) [s[0], s[1], len(s)==2 ? 0 : s[2]]];
+    freqs = len(sines_)>0? [for (i=sines_) i[1]] : [5];
     points = [
         for (a = [0 : (360/segs(r)/max(freqs)) : 360])
-            let(nr=r+sum_of_sines(a,sines)) [nr*cos(a), nr*sin(a)]
+            let(nr=r+sum_of_sines(a,sines_)) [nr*cos(a), nr*sin(a)]
     ];
     polygon(points);
 }