diff --git a/README.Rmd b/README.Rmd
index 4f5ce8d..46ae37a 100644
--- a/README.Rmd
+++ b/README.Rmd
@@ -246,6 +246,15 @@ ggpi +
 
 ## Tips and tricks
 
+### Graphics devices
+
+`geom_magnify()` uses masks. This requires R version 4.1.0 or higher, and a
+graphics device that supports masking. If you are using knitr, you may have
+luck with the `ragg_png` device (which was used to create this README). If your
+device doesn't support masks, only `shape = "rect"` will work, and the plot
+inset will not be clipped to the panel area.
+
+
 ### Adding layers to the inset
 
 `geom_magnify()` stores the plot when it is added to it. So, order matters:
diff --git a/README.md b/README.md
index 3da1434..9c9e60e 100644
--- a/README.md
+++ b/README.md
@@ -250,6 +250,14 @@ ggpi +
 
 ## Tips and tricks
 
+### Graphics devices
+
+`geom_magnify()` uses masks. This requires R version 4.1.0 or higher,
+and a graphics device that supports masking. If you are using knitr, you
+may have luck with the `ragg_png` device (which was used to create this
+README). If your device doesn’t support masks, only `shape = "rect"`
+will work, and the plot inset will not be clipped to the panel area.
+
 ### Adding layers to the inset
 
 `geom_magnify()` stores the plot when it is added to it. So, order