diff --git a/README.md b/README.md
index 8dd0971..1ab2d7c 100644
--- a/README.md
+++ b/README.md
@@ -85,9 +85,9 @@ This function allows you to define a custom basis. `bname bx by bz` must be repe
 
 **efill**
 ```
-efill
+efill xyz
 ```
-This command will rerun the creation algorithm with multiple times starting with an esize of `esize` and decreasing it by half on every iteration in an effort to maximize the reduction of degrees of freedom in the system.
+This command will rerun the creation algorithm with multiple times starting with an esize of `esize` and decreasing it by half on every iteration in an effort to maximize the reduction of degrees of freedom in the system. You must specify which dimensions will be filled. The code accepts `x`, `y`, `z`, `xy`, `yz`, `xz`, and `xyz` specifying which boundaries to fill in.
 
 ### Mode Convert