Which Proto-Indo-European consonant changes occurred in Grimm’s law?

Grimm’s law, also known as the Germanic consonant shift, describes a systematic transformation of Proto-Indo-European stop consonants into the Germanic languages. This shift involved three primary changes to the 12 original Proto-Indo-European stops:12

• Voiceless stops to voiceless fricatives: The Proto-Indo-European voiceless stops p, t, k, and kʷ shifted to the Proto-Germanic voiceless fricatives f, þ (th), x (h), and xʷ (hw).2
• Voiced stops to voiceless stops: The Proto-Indo-European voiced stops b, d, g, and gʷ shifted to the Proto-Germanic voiceless stops p, t, k, and kʷ.2
• Voiced aspirated stops to voiced stops/fricatives: The Proto-Indo-European voiced aspirated stops bh, dh, gh, and gʷh shifted to Proto-Germanic voiced fricatives (ƀ, ð, ǥ, ǥʷ), which later developed into the voiced stops b, d, and g in many positions.2

Exceptions to these patterns, such as the voicing of fricatives following unaccented syllables, were later explained by Verner’s law.3