diff --git a/Technotes/HiddenPrefs.md b/Technotes/HiddenPrefs.md
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--- a/Technotes/HiddenPrefs.md
+++ b/Technotes/HiddenPrefs.md
@@ -2,4 +2,15 @@
 
 See AppDefaults.swift for the app’s preference keys, including hidden prefs.
 
-(TODO: document them here.)
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+Hidden preferences tend to be named for a person:
+
+1. To distinguish them from regular, un-hidden preferences, and
+2. So we can remember who asked for them.
+
+#### Main window titles
+
+The main window doesn’t display its title, partly because it looks cooler that way and partly because it’s redundant information.
+
+The downside to this is that title-less windows do not allow the toolbar to show button names. (This is an AppKit thing.)
+
+To turn window titles on, set `KafasisTitleMode` to true.
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