diff --git a/.gitignore b/.gitignore
index adf8f72..e620e5a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+.DS_Store
+
 # ---> Go
 # If you prefer the allow list template instead of the deny list, see community template:
 # https://github.com/github/gitignore/blob/main/community/Golang/Go.AllowList.gitignore
diff --git a/chapter13/concurrency.go b/chapter13/concurrency.go
new file mode 100644
index 0000000..7a7cc06
--- /dev/null
+++ b/chapter13/concurrency.go
@@ -0,0 +1,30 @@
+package main
+
+import (
+	"fmt"
+)
+
+func a(c chan bool) {
+	for i := 0; i < 50; i++ {
+		fmt.Print("a")
+	}
+
+	c <- true
+}
+
+func b(c chan bool) {
+	for i := 0; i < 50; i++ {
+		fmt.Print("b")
+	}
+
+	c <- true
+}
+
+func main() {
+	c := make(chan bool)
+	go a(c)
+	go b(c)
+	<- c
+	<- c
+	fmt.Println("end main()")
+}