Hi Katharina,
oh, yes, now I really do understand what you mean by "Platt": the (central
an upper German) dialect you remember from your youth in the villages!
So you're partly right: Even some dialect speakers in Germany call their
local slang "Platt". This is correct. But the word "Platt" for dialect is
only used among some Central German dialects. The upper German dialects
don't use this word for their slang.
But: You have to distinguish "Platt" as the shortening of "Plattdeutsch" and
"Platt" as the German word for "dialect", a word which comes from the Greek
language.
So the real Plattdeutsch in fact is a *language* as well as High German is.
Both have the same origin in the dialects of the Western Germanic tribes.
These tribes were classified as "Nordseegermanen" (the later Frisians,
Saxons, Angles, Jutes), "Weser-Rhein-Germanen" (the later Franks, Hessians)
and "Elbgermanen" (the later Lombards, southern Thuringians -> Franconians,
Bavarians, Alemannians).
Exactly the area where the North Sea Germanic tribes settled for centuries
is what until today shows the distribution of Plattdeutsch (Westfalen,
Niedersachsen, Bremen, Hamburg, Schleswig-Holstein / Westphalia, Lower
Saxony, B., H., S-H).
The area of the "Weser-Rhine tribes" is the current Central Germany
(Nordrhein, Pfalz, Saarland, Hessen / Nordrhine area, Palatinate, Saarland,
Hesse, Thuringia), and the "Elbe tribes" migrated south and settled in
Franconia, Bavaria, Austria, Baden-Wuerttemberg, Switzerland an the Alsace,
that's all Upper Germany.
Around 800 A.D. already existed a clear segregation between Old Low German
and Old High German (consisting of Central and Upper German dialects).
Later, in the so-called "2. Germanische Lautverschiebung" (2nd Germanic
phonetical change) this segregation became one big step clearer; then
followed the Middle High German period from 1150 A.D. until about 1500 A.D..
Already since about 1200 A.D. the Middle Low Frankish took its first steps
to develop to a language, the current Dutch.
Then, starting about in the Reformation time, followed the New High German
period which still lasts on.
Greetings / Viele Gr��e,
J�rgen