Possibility \#1

Possibility #1

When a word or particle ends in a vowel,

the following consonant is generally reduplicated.

There are thousands of such examples.

e.g. pañca + khanda > pañcakkhandha

In Kaccāyana this is called (28) para dvebhāvo ṭhāne and

(29) vagge ghosāghosānaṃ tatiyapaṭhamā.

vowel + k

apa	kanta	apakkanta	pp. departed, left
abhi	kamati	abhikkamati	pr. comes out
upa	kilesa	upakkilesa	masc. impurity
ava > o	kanti	okkanti	fem. conception, coming down

vowel + kh

pañca	khandha	pañcakkhandha	masc. five aggregates
taṇhā	khaya	taṇhākkhaya	masc. destruction of craving
ni	khitta	nikkhitta	pp. discarded
pura	khata	purakkhata	pp. preferred; lit. made in front
vowel + g
a	gahesi	aggahesi	aor. took, grabbed
vi	gaha	viggaha	masc. quarrel, argument
catu	guṇa	catugguṇa	adj. folded four times

vowel + gh

pa	gharanta	paggharanta	prp. dripping
sakuṇa	ghi	sakuṇagghi	masc. hawk; lit. bird killer
eka	ghana	ekagghana	adj. dense; lit. one mass

vowel + c

na	cuta	accuta	adj. everlasting
pari	caja	pariccaja	masc. boundary
a + pari	catta	apariccatta	pp. not given up

vowel + ch

vigata	chanda	vigatacchanda	adj. free from desire
seta	chatta	setacchatta	nt. white umbrella

vowel + j

pa	jalati	pajjalati	pr. burns
puthu	jana	puthujjana	masc. ordinary man

vowel + jh

pa	jhāyati	pajjhāyati	pr. contemplates, thinks
abhi	jhā	abhijjhā	f. covetousness

vowel + ñ

sabba	ñū	sabbaññū	adj. all knowing
vi	ñāṇa	viññāṇa	nt. consciousness

vowel + ṭh

anu	ṭhita	anuṭṭhita	pp. established
upa	ṭhapeti	upaṭṭhapeti	pr. applies
adhi	ṭhāna	adhiṭṭhāna	nt. determination
cira	ṭhitika	ciraṭṭhitika	adj. long standing

vowel + ḍ

ni	ḍeti	niḍḍeti4	pr. weeds out

(commentary example)

vowel + t

cha	tiṃsati	chattiṃsati	fem. thirty-six
catu	tāla	catuttāla	adj. four palm trees in height
pari	yasati	parittasati	pr. worries

vowel + th

na	thaddha	atthaddha	adj. not stubborn
vi	thāra	vitthāra	m. breadth
anu	thunāti	anutthunāti	pr. wails, moans
o	tharati	ottharati	pr. spreads over

vowel + d

a	dasā	addasā	imperf. saw
catu	disā	catuddisā	fem. four directions

vowel + dh

anu	dhaṃseti	anuddhaṃseti	pr. attacks
sovaṇṇa	dhaja	sovaṇṇaddhaja	adj. with golden flags

vowel + n

ku	nadī	kunnadī	fem. stream

vowel + p

na	pamāda	appamāda	m. carefulness
ati	pago	atippago	adv. very early
āyu	pamāṇa	āyuppamāṇa	nt. length of life

vowel + ph

pati	pharati	paṭippharati	pr. blasts back
pa	phoṭetvā	papphoṭetvā	abs. having shaken out

after a vowel, v can become bb

vowel + v

giri	vaja	giribbaja	nt. mountain cave
cha	vassa	chabbassa	nt. six years

vowel + bh

a	bhaya	abbhaya	adj. without fear
puna > pono	bhavika	ponobbhavika	adj. leading to repeated existence

vowel + m

ku	magga	kummagga	masc. bad road; wrong way

vowel + y

se	yathā’pi	seyyathā’pi	ind. just like

vowel + l

vijju	latā	vijjullatā	fem. forks of lightning
ava > o	lujjati	ullujjati	pr. is breaking down

vowel + s

pa	sasati	passasati	pr. exhales, breathes out
diṭṭha	suta	diṭṭhassuta	nt. what is seen and heard
anu	saritar	anussaritar	masc. who recollects

Counter-examples:

ajāta + sattu > ajātasattu

upa + pajjati > upapajjati

pa + dāleti > padāleti