voiceless stops change into voiceless fricatives.
Proto-Indo-European voiced stops become voiceless stops.
Proto-Indo-European voiced
aspirated stops become voiced stops or fricatives (as allophones).The chain shift can be abstractly represented as:
bʰ → b → p → ɸ
dʰ → d → t → θ
gʰ → g → k → x
gʷʰ → gʷ → kʷ → xʷ
Grimm’s law