Speakers of Germanic differentiated from the remaining Indo-European speakers with the adoption of the first sound change. All languages, past or present, that exhibit this sound change are members of the Germanic family.

The first sound change

Indo-europeanbecameGermanic
pf
tth
gk
kchh
dt

Examples from English

Latin or GreekEnglishGerman
paterfatherVater
ped/podfootFuss
plat(eau)flatflach
pyrefireFeuer
trin(ity)threedrei
genus genukin kneeKnie
kard/cordheartHertz
This differentiation is thought, on the evidence of river and natural feature names, to have occurred in the region of historical Thuringia and Saxony. From this core area the Germanic spread to the north coast and the foot of the Danish peninsula and eventually to all of Scandinavia except where blocked by Lapp and Finnish speakers.

On the mainland expansion to the east was blocked by the Baltic-speaking tribes and the ancestors of the Finns and the Estes. Expansion into what are now the Czech Republic, Austria and Hungary was slowed by peoples speaking the Illyrian languages.

Along the upper Danube and the upper Rhine, Celtic speaking peoples were dominant and beginning their linguistic expansion.