Grimm’s law

Grimm’s law exists to describe the sound changes that occurred between Proto-Indo-European and Proto-Germanic. Basically it affected a series of stops, where PIE aspirated voiced stops became unaspirated in PG; PIE unaspirated voiced stops became voiceless; and PIE voiceless stops became fricative. The table below shows the affected sounds: the PIE phonemes are represented by columns 1, 2 and 3, and each sound moved one column to the right in the transition to Proto-Germanic:

1	2	3	4
bʰ	b	p	f
dʰ	d	t	θ
gʰ	g	k	x
gʷʰ	gʷ	kʷ	xʷ

It was later realised that Grimm’s law by itself didn’t fully explain the consonantal changes, so it was followed up with Verner’s law.