Indo-European sound laws

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As the Proto-Indo-European language (PIE) broke up, its sound system diverged as well, as evidenced in various sound laws associated with the daughter Indo-European languages.

Linguistic aspect of the Proto-Indo-European languages

Especially notable is the palatalization that produced the satem languages, along with the associated ruki sound law. Other notable changes include:

Grimm's law and Verner's law in Proto-Germanic
an independent change similar to Grimm's law in Armenian
loss of prevocalic *p- in Proto-Celtic
Brugmann's law in Proto-Indo-Iranian
Winter's law and Hirt's law in Balto-Slavic
merging of voiced and breathy-voiced stops, and /a/ and /o/, in various "northern" languages

Bartholomae's law in Indo-Iranian, and Sievers's law in Proto-Germanic and (to some extent) various other branches, may or may not have been common Indo-European features. A number of innovations, both phonological and morphological, represent areal features common to the Italic and Celtic languages; among them the development of labiovelars to labial consonants in some Italic and Celtic branches, producing "p-Celtic" and "q-Celtic" languages (likewise "p-Italic" and "q-Italic", although these terms are less used). Another grouping with many shared areal innovations comprises Greek, Indo-Iranian, and Armenian; among its common phonological innovations are Grassmann's law in Greek and Indo-Iranian, and weakening of pre-vocalic /s/ to /h/ in Greek, Iranian and Armenian.

Consonants

The following table shows the Proto-Indo-European consonants and their reflexes in selected Indo-European daughter languages. Background and further details can be found in various related articles, including Proto-Indo-European phonology, Centum and satem languages, the articles on the various sound laws referred to in the introduction, and the articles on the various IE proto-languages, language groups and language phonologies. For development of the laryngeals and syllabic consonants, see the vowels table below.

Table 1. Reflexes of Proto-Indo-European single consonants
PIE	Indo-Iranian		Balto-Slavic		Alb.	Arm.	Anatol.	Toch.	Greek		Italic	Celtic		Germanic
	Sanskrit	Avestan	O.C.S.	Lith.			Hitt.		Greek		Latin	Old Irish		Gothic	English
	Sanskrit	Avestan	O.C.S.	Lith.			Hitt.		normal	C+[j] ^{[C 1]}	Latin	normal	-C- ^{[C 2]} ^{[C 1]}	Gothic	English
*p	p; ph [pʰ] ^{[C 3]}	p; f ^{[C 4]}	p			h; w ^{[C 5]}	p, pp	p		pt	p	∅		f; b [β] ^{[C 6]}	f; v, f^{[C 2]}
*t	t; th [t̪ʰ] ^{[C 3]}	t; θ^{[C 4]}	t			tʿ [tʰ]	t, tt; z [ts] ^{[C 7]}	t; c [c] ^{[C 7]}	t	s; tt/ss^{[C 5]}	t	t	th [θ]	þ [θ]; d [ð]; ^{[C 6]}	th; d; ^{[C 6]}
*ḱ	ś [ɕ]	s		š [ʃ]	th [θ]; k^{[C 8]}	s	k, kk	k; ś [ɕ]^{[C 8]}	k		c [k]	c [k]	ch [x]	h; g [ɣ] ^{[C 6]}	h; ∅;^{[C 2]} y ^{[C 6]}
*k	k; c [t͡ɕ]; ^{[C 7]} kh [kʰ] ^{[C 3]}	k; c [tʃ]; ^{[C 7]} x^{[C 4]}	k; č [tʃ]; ^{[C 7]} c [ts]^{[C 9]}	k	k; q [c]^{[C 9]}	kʿ [kʰ]	k, kk		k		c [k]			h; g [ɣ] ^{[C 6]}	h; ∅;^{[C 2]} y ^{[C 6]}
*kʷ	k; c [t͡ɕ]; ^{[C 7]} kh [kʰ] ^{[C 3]}	k; c [tʃ]; ^{[C 7]} x^{[C 4]}	k; č [tʃ]; ^{[C 7]} c [ts]^{[C 9]}	k	k; s; ^{[C 7]} q [c]^{[C 9]}	kʿ [kʰ]	ku, kku		p; t; ^{[C 7]} k^{[C 10]}		qu [kʷ]; c [k] ^{[C 11]}			ƕ [ʍ]; gw, w ^{[C 6]}	wh; w ^{[C 6]}
*b	b; bh ^{[C 3]}	b; β ^{[C 12]}	b			p			b	pt	b	b [b]	-[β]-	p
*d	d; dh ^{[C 3]}	d; δ ^{[C 12]}	d		d; dh [ð]^{[C 2]}	t		ts; ś [ɕ] ^{[C 7]}	d	z [zd] > [z]	d	d [d]	-[ð]-	t
*ǵ	j [d͡ʑ]; h [ɦ] ^{[C 3]}	z		ž [ʒ]	dh [ð]; g^{[C 8]}	c [ts]	k	k; ś [ɕ]^{[C 8]}	g		g	g [ɡ]	-[ɣ]-	k	c / k; ch^{[C 9]}
*g	g; j [d͡ʑ]; ^{[C 7]} gh; ^{[C 3]} h [ɦ] ^{[C 3]}	g; j [dʒ]; ^{[C 7]} γ ^{[C 12]}	g; ž [ʒ]; ^{[C 7]} dz^{[C 9]}	g	g	k	k		g		g	g [ɡ]	-[ɣ]-	k	c / k; ch^{[C 9]}
*gʷ	g; j [d͡ʑ]; ^{[C 7]} gh; ^{[C 3]} h [ɦ] ^{[C 3]}	g; j [dʒ]; ^{[C 7]} γ ^{[C 12]}	g; ž [ʒ]; ^{[C 7]} dz^{[C 9]}	g	g; z; ^{[C 7]} gj [ɟ]^{[C 9]}	k	ku		b; d; ^{[C 7]} g^{[C 10]}		u [w > v]; gu [ɡʷ] ^{[C 13]}	b [b]	-[β]-	q [kʷ]	qu
*bʰ	bh [bʱ]	b; β ^{[C 12]}	b			b; w^{[C 2]}	p		ph [pʰ]	pt	f;^{[C 14]} b	b [b]	-[β]-	b [b]; b [β];^{[C 2]} f ^{[C 15]}	b; v / f^{[C 16]}
*dʰ	dh [dʱ]	d; δ ^{[C 12]}	d				t	t; c [c] ^{[C 7]}	th [tʰ]	tt/ss	f;^{[C 14]} d; b ^{[C 17]}	d [d]	-[ð]-	d; d [ð];^{[C 2]} þ ^{[C 15]}	d
*ǵʰ	h [ɦ]	z		ž [ʒ]	dh [ð]; d^{[C 8]}	j [dz]; z^{[C 2]}	k	k; ś [ɕ] ^{[C 7]}	kh [kʰ]		h; h / g^{[C 8]}	g [ɡ]	-[ɣ]-	g; g [ɣ];^{[C 2]} g [x] ^{[C 15]}	g; y / w^{[C 16]}
*gʰ	gh [ɡʱ]; h [ɦ] ^{[C 7]}	g; j [dʒ]; ^{[C 7]} γ ^{[C 12]}	g; ž [ʒ]; ^{[C 7]} dz^{[C 9]}	g	g	g; ǰ [dʒ] ^{[C 7]}	k		kh [kʰ]		h; h / g^{[C 8]}			g; g [ɣ];^{[C 2]} g [x] ^{[C 15]}	g; y / w^{[C 16]}
*gʷʰ	gh [ɡʱ]; h [ɦ] ^{[C 7]}	g; j [dʒ]; ^{[C 7]} γ ^{[C 12]}	g; ž [ʒ]; ^{[C 7]} dz^{[C 9]}	g	g; z; ^{[C 7]} gj [ɟ]^{[C 9]}	g; ǰ [dʒ] ^{[C 7]}	ku		ph [pʰ]; th [tʰ]; ^{[C 7]} kh [kʰ]^{[C 10]}		f;^{[C 14]} g / u [w];^{[C 2]} gu [ɡʷ] ^{[C 13]}			g; b;^{[C 14]} w;^{[C 2]} gw ^{[C 13]}	g; b;^{[C 14]} w^{[C 2]}
*s	s	h [h, x]	s		sh [ʃ]; gj [ɟ];^{[C 18]} h^{[C 2]}	h; ∅^{[C 2]}	š [s]	s; ṣ [ʂ]	h;^{[C 14]} s;^{[C 19]}^{[C 15]}/ ∅;^{[C 2]} [¯] ^{[C 20]}	i	s; r^{[C 2]}	s ʃ	-[h]-	s; z ^{[C 6]}	s; r ^{[C 6]}
*s	ṣ [ʂ]^{[C 21]}	š [ʃ]^{[C 21]}	x [x]^{[C 21]}	š [ʃ]^{[C 21]}	sh [ʃ]; gj [ɟ];^{[C 18]} h^{[C 2]}	h; ∅^{[C 2]}	š [s]	s; ṣ [ʂ]	h;^{[C 14]} s;^{[C 19]}^{[C 15]}/ ∅;^{[C 2]} [¯] ^{[C 20]}	i	s; r^{[C 2]}	s ʃ	-[h]-	s; z ^{[C 6]}	s; r ^{[C 6]}
*m	m									in	m	m [m]	-[w̃]-	m
*-m ^{[C 15]}	m		˛ [˜]	n		∅	n	n		--	m [˜]	n		∅
*n	n		n; ˛ [˜] ^{[C 15]}	n				n; ñ [ɲ]	n	in	n
*l	r (dial. l)	r	l		l; ll [ɫ]^{[C 2]}	l / ɫ [ɫ > ɣ]	l			il	l
*r	r/l^{[C 22]}	r			r [ɾ]; rr [r]^{[C 2]}	r				ir	r
*y	y [j]		j [j]		gj [ɟ]; ∅	∅	y [j]		z [zd] > [z] / h; ∅ ^{[C 2]}	?i	i [j]; ∅ ^{[C 2]}	∅		j	y
*w	v [ʋ]	v [w]	v	v [ʋ]	v	g / w	w		w > h / ∅	i	u [w > v]	f	-∅-	w
PIE	Skr.	Av.	O.C.S.	Lith.	Alb.	Arm.	Hitt.	Toch.	Greek	Greek+/j/	Latin	Old Irish		Gothic	English

Notes for table 1:

^ ^a ^b A capital C stands for consonant in this table
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q ^r ^s ^t Between vowels
^ ^a ^b ^c ^d ^e ^f ^g ^h Before an original h₂.
^ ^a ^b ^c Before a consonant or original laryngeal.
^ ^a ^b After a vowel.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Following an unstressed vowel (Verner's law).
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q ^r ^s ^t ^u Before a (PIE) front vowel (*i, *e).
^ ^a ^b ^c ^d ^e ^f Before a sonorant.
^ ^a ^b ^c ^d ^e ^f ^g ^h Before secondary (post-PIE) front-vowels.
^ ^a ^b ^c Before or after a (PIE) u
^ Before or after a (PIE) rounded vowel (*u, *o).
^ ^a ^b ^c ^d ^e ^f In Younger Avestan, after a vowel.
^ ^a ^b ^c After n.
^ ^a ^b ^c ^d ^e ^f At the beginning of a word
^ ^a ^b ^c ^d ^e ^f At the end of a word.
^ ^a ^b Between vowels, or between a vowel and r, l (on either side)
^ After u, r or before r, l.
^ Before a stressed vowel
^ Before or after an obstruent (p, t, k, etc.; s)
^ Before or after a resonant (r, l, m, n).
^ ^a ^b ^c ^d After r, u, k, i (Ruki sound law).
^ rare

Consonant clusters

Proto-Indo-European also had numerous consonant clusters, such as *st, *ḱs. In most cases in most languages, each consonant in a cluster develops according to the normal development given in the table above. Many consonant clusters however also show special developments in multiple languages. Some of these are given by the following table (with cases of otherwise predictable development in gray):

Table 2. Reflexes of Proto-Indo-European consonant clusters
PIE	Indo-Iranian		Balto-Slavic		Alb.	Arm.	Anatol.	Toch.	Greek		Italic	Celtic	Germanic
	Skr.	Av.	O.C.S.	Lith.			Hitt.		Greek		Latin	Old Irish	Gothic	English
	Skr.	Av.	O.C.S.	Lith.			Hitt.		normal	C+[j]	Latin	Old Irish	Gothic	English
*sr	sr	r	str	sr	rr [r]	(a)r	š(ša)r ʃʃr		rh	n/a	fr-, -br-	sr	str	str
*tw	tv	θβ	tv	tv	t	kʿ [kʰ]	ttu, ddu		s-, -ss-	n/a	p?	t	þw	thw
*dʰw	dhv	ðβ	dv	dv	d	?	tu, du				f	d	dw	dw
*dw	dv	ðβ	dv	dv	d	(e)rk	tu, du				b	d	tw	tw
*tl			sl	kl, sl					tl		kl	tl
*dl	ll
*dn	nn, ṇṇ
*ḱw	śv	sp	sv	šv	s	sk, š				n/a	qu [kʷ]	cu [kʷ]	ƕ [xʷ]	wh
*ǵʰw	hv		zv	žv	z
*ǵw	jv		zv	žv	z								q [kʷ]	qu [kw]
*sw	sv	xuu [xʷ]	sv	sv	v, d^{[CC 1]}	kʿ [kʰ]	normal dev.		h	n/a	su [sw]	s	sw	sw
*sp	sp	sp ^{[CC 2]}	sp		f	sp ^{[CC 2]} / pʿ [pʰ]	normal dev.		sp ^{[CC 2]}	?	sp	f	sp ^{[CC 3]}
*sbʰ		spʰ
*sd	d; ḷ^{[CC 4]} > ḍ, ḍḍ	zd	zd	zd		st					d	-t- [d]	st	st
*sdʰ	dh; ḷh^{[CC 4]} > ḍh, ḍḍh	zd	zd	zd					sth			-t- [d]	zd	d
*st	st; ṣṭ^{[CC 4]}	st ^{[CC 2]}	st		sht [ʃt]	st ^{[CC 2]}	normal dev.		st ^{[CC 2]}	s; tt/ss^{[CC 5]}	st		st ^{[CC 3]}
*sḱ	ch [t͡ɕʰ]; cch^{[CC 5]}	s?	sk	š?	h	?? č`; c`^{[CC 5]}	normal dev.		sk; ^{[CC 2]} kh [kʰ];^{[CC 6]} skh [skʰ] ^{[CC 7]}		sc [sk]	sc [sk]	sk ^{[CC 3]}	sh [ʃ]
*sk	sk, {śc, ch}^{[CC 8]}	sk, sč ^{[CC 2]}	normal dev.		?	sk ^{[CC 2]}	normal dev.		sk; ^{[CC 2]} kh [kʰ];^{[CC 6]} skh [skʰ] ^{[CC 7]}		sc [sk]		sk ^{[CC 3]}
*skʷ	sk, {śc, ch}^{[CC 8]}	sk, sč ^{[CC 2]}	normal dev.		?	sk ^{[CC 2]}	normal dev.		norm.		squ [skʷ]		sq ^{[CC 3]}
*t+t [tst]	tt; tth ^{[CC 9]}	st; sθ^{[CC 10]}?	st		s	s?	zt, zzašt, zzazz [tst]	ss?	st		ss		ss / st
*sǵ	jj
*sgʰ	jj^{[CC 8]}
*dt	tt	st						st	st		s		ss	ss [s]
*ddʰ	ddʰ	zd									d	t
*dʰt	ddʰ	zd, st		st					st
*pt	pt	ft	t?	pt	t				pt		pt	cht [xt]	ft ^{[CC 3]}
*ḱt	ṣṭ [ʂʈ]	št [ʃt]	st	št [ʃt]					kt		ct [kt]		ht ^{[CC 3]}	ght [t] ^{[CC 3]}
*kt	kt	xt	t?	kt					kt		ct [kt]
*kʷt	kt	xt	t?	kt					pt		ct [kt]
*ps	ps								ps		ps	s, ss	fs	ps
*ts	ts
*ḱs	kṣ	š			sh [ʃ]				ks		x [ks]		hs	x [ks]
*ks	kṣ^{[CC 4]}	xš^{[CC 4]}						(ks)	ks		x [ks]		hs
*kʷs	kṣ^{[CC 4]}	xš^{[CC 4]}					kʷs		ps		x [ks]
*gs	kṣ
*gʷs	kṣ
*ǵʰs	kṣ	ž								ks
*gʰs	kṣ
*gʷʰs	kṣ								ps
*tḱ	kṣ	š			k				kt		s
*tk	kṣ	xš							kt
*dʰǵʰ	kṣ	z		ž			tk	tk/k	khth
*dʰgʰ	kṣ						tk
*dʰgʷʰ	kṣ	γž					kts		phth		s
PIE	Skr.	Av.	O.C.S.	Lith.	Alb.	Arm.	Hitt.	Toch.	Greek	Greek+/j/	Latin	Old Irish	Gothic	English

Notes to Table 2:

^ Word initially. See Orel, V. (2000).
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ In contrast to *s normally giving h in Avestan, Armenian and Greek.
^ ^a ^b ^c ^d ^e ^f ^g Germanic spirant law: *p, *t, *k, *kʷ remain stops when preceded by another stop or *s.
^ ^a ^b ^c ^d ^e After r, u, k, i (Ruki sound law).
^ ^a ^b ^c Between vowels.
^ After r, l, m, n, t, d, possibly other consonants?
^ After (Greek) th causes aspiration in the cluster and then disappears, typical example *πάθ-σκ-ω (*páth-sk-ō) gives πάσχω (páskhō). ^{[clarification needed]}
^ ^a ^b Before front vowels.
^ Before an original laryngeal.
^ Before a consonant or original laryngeal.

Vowels and syllabic consonants

This table shows the Proto-Indo-European vowels and syllabic consonants (as reconstructed both before and after the acceptance of laryngeal theory), and their reflexes in selected Indo-European daughter languages. Background and further details can be found in various related articles, including Proto-Indo-European phonology, the articles on the various sound laws referred to in the introduction, and the articles on the various IE proto-languages, language groups and language phonologies.

Trad. PIE	Laryng. PIE	Skr.	Av.	O.C.S.	Lith.	Arm.	Alb.	Toch.	Hitt.	Greek	Latin^{[V 1]}	Proto-Celtic	Gothic^{[V 2]}	Old English^{[V 1]}
Trad. PIE	Laryng. PIE	Skr.	Av.	O.C.S.	Lith.	Arm.	Alb.	Toch.	Hitt.	Greek	Latin^{[V 1]}	Proto-Celtic	Gothic^{[V 2]}	normal	umlauted^{[V 3]}
*e	e, h₁e	a		e			je, ie, e, i; ja^{[V 4]}	ä	e, ʔe, i	e			i; aí [ɛ]^{[V 5]}	e; eo^{[V 6]}	i; ie^{[V 6]}
*a	(a^{[V 7]}), h₂e			o	a	a	ha, a	ā	ha, a	a			a	æ; a;^{[V 8]} ea^{[V 6]}	e; ie^{[V 6]}
*o	*h₃e					o, a	a	a, e	a	o
*o	*o	a; ā^{[V 9]}				o, a		a, e	a	o
*ə^{[V 10]}	*h₁^{[V 10]}	i	i, ∅	∅		a, ∅		ā	∅	e	a		a, ∅
	*h₂^{[V 10]}								ḫ	a
	*h₃^{[V 10]}								∅	o
*-	*h₁-^{[V 11]}	∅				e (a?)	∅		a, ∅	e (o)	∅
	*h₂-^{[V 11]}					a			ḫa	a
	*h₃-^{[V 11]}					a			a, ha	o
*ē	ē, eh₁	ā		ě	ė	i	o, ua	a/e?; ā?^{[V 12]}	ē, e, i	ē		ī	ē	ǣ
*ā	(ā^{[V 7]}), eh₂			a	o [oː]	a	o, ua	a/o?	a, ah	ā > ē^{[V 13]}	ā	ā	ō		ē
*ō	ō, eh₃			a	uo	u	e	a/ā?; ū?^{[V 12]}	a, ā	ō		ā; ū^{[V 12]}	ō		ē
*i	*i	i		ь	i	i	i; e^{[V 14]}	ä	i, ī	i			i; aí [ɛ]^{[V 5]}	i
*ī	*ih₁	ī		i	y [iː]	i	i	i		ī	ī		ei [iː]	ī
	*ih₂					i or ^(j)a?^{[V 15]}		yā		ī or ^(j)ā?^{[V 15]}
	*ih₃					i or ^(j)a?^{[V 15]}		yā		ī or ^(j)ō?^{[V 15]}
*ei	ei, h₁ei	ai > ē	ai > ōi, āi > aē^{[V 9]}		ei; ie^{[V 16]}	i		e	ē	ei	ī	ē
*oi	oi, h₃ei			ě	ai; ie^{[V 16]}	e	e, ai		ē, ai	oi	ū	oi	ái	ā	ǣ
*ai	(ai^{[V 7]}), h₂ei			ě	ai; ie^{[V 16]}	e	e, ai		ē, ai	ai	ae	ai	ái	ā	ǣ
*ēi	*ēi	āi > ai; ā^{[V 12]}	āi; ā(i)^{[V 12]}	i			i		ē	ēi	ī?		ei [iː]	ī
*ōi	ōi (oei)			y; u^{[V 12]}	ai; ui^{[V 12]}		e, ai		ai	ōi	ō	u^{[V 12]}	ái	ā	ǣ
*āi	*eh₂ei			ě			e, ai		ai	āi > ēi^{[V 13]}	ae		ái	ā	ǣ
*u	*u	u		ъ	u	u	u; y^{[V 17]}	ä	u	u	u	u; o^{[V 18]}	u; aú [ɔ]^{[V 5]}	u; o^{[V 19]}	y
*ū	*uh₁	ū		y	ū	u	y; i^{[V 12]}	u		ū	ū				ȳ
	*uh₂					u or ^(w)a?^{[V 15]}		wā		ū or ^(w)ā?^{[V 15]}
	*uh₃					u or ^(w)a?^{[V 15]}		wā		ū or ^(w)ō?^{[V 15]}
*eu	eu, h₁eu	au > ō	ə̄u; ao^{[V 9]}	ju	iau	oy	e	u		eu	ū	ou	iu	ēo	īe
*ou	ou,h₃eu			u	au	oy	a	o, au		ou	ū		áu	ēa
*au	(au^{[V 7]}), h₂eu			u	au	aw	a	o, au		au	au		áu	ēa
*ēu	*ēu	āu > au	āu	u	iau		e				ū?		iu	ēo
*ōu	*ōu	āu > au	āu				a		au	ō			áu	ēa
*m̥	*m̥	a		ę	im̃; um̃^{[V 20]}	am	a	äm	a, un	a	em	em, am	um	um	ym
*m̥̅	*mh₁	ā			ìm; ùm^{[V 20]}	ama		mā		mē	mā
	*mh₂									mā > mē^{[V 13]}
	*mh₃									mō
*m̥m	*m̥m	am		ьm/ъm	im; um^{[V 20]}	am				am	em	am
*n̥	*n̥	a		ę	iñ; uñ^{[V 20]}	an		än	an	a	en	en, an	un	un	yn
*n̥̄	*nh₁	ā			ìn; ùn^{[V 20]}	ana		nā		nē	nā
	*nh₂									nā > nē ^{[V 13]}
	*nh₃									nō
*n̥n	*n̥n	an		ьn/ъn	iñ; uñ^{[V 20]}	an				an	en	an
*l̥	*l̥	r̥	ərə	lь/lъ	il̃; ul̃^{[V 20]}	al	il, li; ul, lu	äl	al	la	ol	li;^{[V 21]} al	ul	ul; ol^{[V 19]}	yl
*l̥̄	*lh₁	īr; ūr^{[V 22]}	arə		ìl; ùl^{[V 20]}	ala	al	lā		lē	lā
	*lh₂									lā > lē^{[V 13]}
	*lh₃									lō
*l̥l	*l̥l	ir; ur^{[V 22]}	ar	ьl/ъl	il; ul^{[V 20]}	al, la				al	el	al
*r̥	*r̥	r̥	ərə	rь/rъ	ir̃; ur̃^{[V 20]}	ar	ir, ri; ur, ru	är	ar, ur	ra	or	ri;^{[V 21]} ar	aúr [ɔr]	ur; or^{[V 19]}	yr
*r̥̄	*rh₁	īr; ūr^{[V 22]}	arə		ìr; ùr^{[V 20]}	ara	ra	rā		rē	rā
	*rh₂									rā > rē^{[V 13]}
	*rh₃									rō
*r̥r	*r̥r	ir; ur^{[V 22]}	ar	ьr/ъr	ir; ur^{[V 20]}	ar				ar	ar	ar
Trad. PIE	Laryng. PIE	Skr.	Av.	O.C.S.	Lith.	Arm.	Alb.	Toch.	Hitt.	Greek	Latin^{[V 1]}	Proto-Celtic	Gothic^{[V 2]}	normal	umlauted^{[V 3]}
Trad. PIE	Laryng. PIE	Skr.	Av.	O.C.S.	Lith.	Arm.	Alb.	Toch.	Hitt.	Greek	Latin^{[V 1]}	Proto-Celtic	Gothic^{[V 2]}	Old English^{[V 1]}

Notes:

^ ^a ^b ^c ^d In initial syllables only.
^ ^a ^b In non-final syllables only.
^ ^a ^b Before i, ī, or /j/ in the next syllable in Proto-Germanic (i-umlaut).
^ In a closed syllable.
^ ^a ^b ^c Before r, h. Gothic, but not other Germanic languages, merges /e/ and /i/.
^ ^a ^b ^c ^d Before h, w, or before r, l plus a consonant ("breaking").
^ ^a ^b ^c ^d The existence of PIE non-allophonic a is disputed.
^ Before a back vowel in the next syllable (a restoration).
^ ^a ^b ^c In open syllables (Brugmann's law).
^ ^a ^b ^c ^d Between consonants, or at the end of a word after a consonant.
^ ^a ^b ^c At the beginning of a word, followed by a consonant.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ In a final syllable.
^ ^a ^b ^c ^d ^e ^f ā > ē in Attic and Ionic dialects only.
^ Before ā in the following syllable.
^ ^a ^b ^c ^d ^e ^f The so-called breaking is disputed (typical examples are *proti-h₃kʷo- > Ved. prátīkam ~ Gk. πρόσωπον; *gʷih₃u̯o- > Ved. jīvá- ~ Arm. keank‘, Gk. ζωός; *duh₂ro- > Ved. dūrá- ~ Arm. erkar, Gk. δηρός)
^ ^a ^b Under stress.
^ Before i in the following syllable.
^ Before wa.
^ ^a ^b ^c Before a non-high vowel in the next syllable (a-mutation).
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l In the neighbourhood of labiovelars.
^ ^a ^b Before a stop or m.
^ ^a ^b ^c ^d In the neighbourhood of labials.

Examples

See the list of Proto-Indo-European roots hosted at Wiktionary.

*p

*pṓds, ~ *ped-, "foot".^[1]

Vedic Sanskrit: pā́t, pád-
Avestan: paδa, pāδa
Old Church Slavonic: pěšĭ, "on foot"
Albanian: poshtë, "below"
Lithuanian: pėda, "foot bottom"
Armenian: otn
Tocharian: A pe, B pai
Luwian: pa-da-, pa-ta-
Ancient Greek: poús, podós
Latin: pēs, pedis
Celtiberian: ozas
Gothic: fotus (*p -> f by Grimm's Law)

*t

*tréyes, "three".^[1]

Vedic Sanskrit: tráyas
Avestan: θrāiiō
Old Church Slavonic: trĭje
Lithuanian: trỹs
Albanian: tre
Ancient Greek: treĩs
Latin: trēs
Old Irish: trí
Welsh: tri
Armenian: erekʿ
Gothic: þreis (*t -> þ by Grimm's Law)

*ḱ

*ḱm̥tóm, "hundred" (from earlier *dḱm̥tóm)^[1]

Vedic Sanskrit: śatám
Younger Avestan: satəm
Old Church Slavonic: sŭto
Lithuanian: šim̃tas
Tocharian: A känt, B känte
Ancient Greek: hekatón
Latin: centum
Old Irish: cét
Welsh: cant
Gothic: hund (from Proto-Germanic *hund-)^[2]

*k

*kréwh₂s, "raw flesh"^[1]

Vedic Sanskrit: kravís-, "raw meat"
Lithuanian: kraũjas, "blood"
Old Church Slavonic: kry, "blood"
Ancient Greek: kréas, "meat"
Latin: cruor, "raw blood"
Old Irish: crú, "blood, gore"
Old English: hrǣw, "raw"

Sound laws within PIE

A few phonological laws can be reconstructed that may have been effective prior to the final breakup of PIE by internal reconstruction.

Sievers' law (Edgerton's law, Lindeman's option)
Bartholomae's law
Szemerényi's law
Stang's law
Siebs' law
Weise's law

References

^ ^a ^b ^c ^d Meier-Brügger, Michael; Gertmenian, Charles (translator) (2003). Indo-European linguistics. Berlin [u.a.]: de Gruyter. pp. 101–131. ISBN 3-11-017433-2. {{cite book}}: |first2= has generic name (help)
^ Hock, Hans Heinrich (1986). Principles of historical linguistics. Berlin; New York: Walter de Gruyter. p. 132. ISBN 3-11-010600-0.