Rule for neuter · das
-lein → das, always neuter (a rarer cousin of -chen).
Just like -chen, the suffix -lein forms diminutives, and the result is always neuter (das), no matter what gender the base word originally had. It's an older, less common variant today, typical of southern German-speaking regions or of a more literary, old-fashioned tone (das Fräulein, "young lady," is probably the only -lein word most speakers still recognize). None of our 2,400 most frequent words end in -lein, so this pattern is very rare - but if you ever come across a word formed this way, you can be sure it's das.