diff --git a/layout/svg/base/src/nsISVGChildFrame.h b/layout/svg/base/src/nsISVGChildFrame.h
index 38c73eec57e..bc145493850 100644
--- a/layout/svg/base/src/nsISVGChildFrame.h
+++ b/layout/svg/base/src/nsISVGChildFrame.h
@@ -57,6 +57,13 @@ class SVGLengthList;
 class SVGUserUnitList;
 }
 
+/**
+ * This class is not particularly well named. It is inherited by some, but
+ * not all SVG frame classes that can be descendants of an
+ * nsSVGOuterSVGFrame in the frame tree. Note specifically that SVG container
+ * frames that do not inherit nsSVGDisplayContainerFrame do not inherit this
+ * class (so that's classes that only inherit nsSVGContainerFrame).
+ */
 class nsISVGChildFrame : public nsQueryFrame
 {
 public:
diff --git a/layout/svg/base/src/nsSVGContainerFrame.h b/layout/svg/base/src/nsSVGContainerFrame.h
index 0f53d34945f..7ebdb740626 100644
--- a/layout/svg/base/src/nsSVGContainerFrame.h
+++ b/layout/svg/base/src/nsSVGContainerFrame.h
@@ -47,6 +47,13 @@ class nsRenderingContext;
 
 typedef nsContainerFrame nsSVGContainerFrameBase;
 
+/**
+ * Base class for SVG container frames. Frame sub-classes that do not
+ * display their contents directly (such as the frames for <marker> or
+ * <pattern>) just inherit this class. Frame sub-classes that do or can
+ * display their contents directly (such as the frames for inner-<svg> or
+ * <g>) inherit our nsDisplayContainerFrame sub-class.
+ */
 class nsSVGContainerFrame : public nsSVGContainerFrameBase
 {
   friend nsIFrame* NS_NewSVGContainerFrame(nsIPresShell* aPresShell,
@@ -79,6 +86,10 @@ public:
   }
 };
 
+/**
+ * Frame class or base-class for SVG containers that can or do display their
+ * contents directly.
+ */
 class nsSVGDisplayContainerFrame : public nsSVGContainerFrame,
                                    public nsISVGChildFrame
 {