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Grimm's Law
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The two brothers were in their early twenties when they began the linguistic and philological studies that would culminate in both Grimm's Law and their collected editions of fairy and folk tales. Though their collections of tales became immensely popular, they were essentially a byproduct of the linguistic research which was the Brothers' primary goal.
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The Grimm brothers invented Grimm's Law, which tried to prove that Germans are more studious, pedantic, and phlegmatic --- ... more grim --- than anybody else. When the Grimms weren't inventing laws, they spent their evening mixing pleasantly with peasants (which they mistakenly called "pheasants") --- collecting lore to put in their books.
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Most other Indo-European consonants, — /L/, /M/, /N/, /R/, and /S/ in particular — were not affected by Grimm's Law. Observe that all these are the "extended" consonants — those that can be drawn out for a long time. (Mmmmmm…, ssssss…, etc.) Grimm's Law seems to have only affected the "plosive" sounds that cannot be prolonged.
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Grimm's Law