diff --git a/en/03_Drawing_a_triangle/01_Presentation/01_Swap_chain.adoc b/en/03_Drawing_a_triangle/01_Presentation/01_Swap_chain.adoc
index 5047177f..eef800a6 100644
--- a/en/03_Drawing_a_triangle/01_Presentation/01_Swap_chain.adoc
+++ b/en/03_Drawing_a_triangle/01_Presentation/01_Swap_chain.adoc
@@ -285,6 +285,9 @@ matching it against the minimum and maximum image extent.
 
 vk::Extent2D chooseSwapExtent(vk::SurfaceCapabilitiesKHR const &capabilities)
 {
+    // currentExtent is only set to the special "undefined" value described above
+    // when the window manager lets us choose the extent ourselves; any other value
+    // means the surface already dictates a fixed extent that we must use as-is.
     if (capabilities.currentExtent.width != std::numeric_limits<uint32_t>::max())
     {
         return capabilities.currentExtent;