diff --git a/21st_century_c/chapter11/Makefile b/21st_century_c/chapter11/Makefile
index 1087630..74dfa6c 100644
--- a/21st_century_c/chapter11/Makefile
+++ b/21st_century_c/chapter11/Makefile
@@ -1,20 +1,18 @@
 CFLAGS = -Wall -O3 --std=c11
 
 
-boxes : boxes.c
-	$(CC) $(CFLAGS) $@.c -o $@
-
-show_tree : process_dir.h process_dir.c show_tree.c
-	$(CC) $(CFLAGS) process_dir.c $@.c -o $@
-
 simple_cplx : cplx.h complex.c simple_cplx.c
 	$(CC) `pkg-config --cflags --libs gsl` $(CFLAGS) complex.c $@.c -o $@
 
 seamlessone : seamlessone.c
 	$(CC) $(CFLAGS) -fms-extensions -Wno-microsoft $@.c -o $@
 
+seamlesstwo : seamlesstwo.c
+	$(CC) $(CFLAGS) -fms-extensions -Wno-microsoft $@.c -o $@
+
+
 .PHONY : all
-all : boxes show_tree simple_cplx
+all : simple_cplx seamlessone seamlesstwo
 
 .PHONY : check
 check : *.h *.c
@@ -22,8 +20,7 @@ check : *.h *.c
 
 .PHONY : clean
 clean :
-	rm -f boxes
-	rm -f show_tree
 	rm -f simple_cplx
 	rm -f seamlessone
+	rm -f seamlesstwo
 	rm -rf *.dSYM
diff --git a/21st_century_c/chapter11/seamlesstwo.c b/21st_century_c/chapter11/seamlesstwo.c
new file mode 100644
index 0000000..d5c9b1a
--- /dev/null
+++ b/21st_century_c/chapter11/seamlesstwo.c
@@ -0,0 +1,38 @@
+#include <stdio.h>
+#include <math.h>
+
+
+
+typedef struct point {
+  double x, y;
+} point;
+
+
+
+typedef struct {
+  union {
+    struct point;
+    point p2;
+  };
+  double z;
+} threepoint;
+
+
+double length(point p) {
+  return sqrt(p.x * p.x + p.y * p.y);
+}
+
+double threelength(threepoint p) {
+  return sqrt(p.x * p.x + p.y * p.y + p.z * p.z);
+}
+
+
+int main() {
+
+  threepoint p = {.x = 3, .y = 0, .z = 4};
+  printf("p is %g units from the origin\n", threelength(p));
+
+  double xylength = length(p.p2);
+  printf("Its projection onto the XY plane is %g units from the origin\n", xylength);
+
+}
\ No newline at end of file