Turkish makes use of three buffer letters -n- -y- -s-
This is to keep vowels apart when adding suffixes.

The buffer letter -S-

Only used with the third person suffix -ı -i -u -ü he, she, it

It becomes -si -sı -sü -su when added to a root word which ends in a vowel.

It is not used in any other instance.

Turkish Buffer Letter -S-
Simple NounMeaningSuffixedMeaning
kedi catkedisi
his / her cat
şemsiye umbrella şemsiyesi
his / her umbrella
kapı doorkapısı
his / her / its door
palto overcoatpaltosu
his /her overcoat
ütü flatironütüsü
his /her iron
araba cararabası
his / her car
kuzu lambkuzusu
his / her lamb

Turkish buffer letter -Y-

Buffer letter -Y- is only used with simple nouns

Buffer -Y is used ON PLAIN SIMPLE UNSUFFIXED NOUNS with the direct object Condition suffix -i the

It becomes -yi -yı -yu -yü when added to a root word which ends in a vowel.

kedi catkediyi
the cat [obj.]

banka bankbankayı
the bank [obj.]

Buffer -Y is used ON PLAIN SIMPLE UNSUFFIXED NOUNS with the the Movement Toward Condition [dative] -a, -e to, towards

It becomes -ya -ye when added to a root word which ends in a vowel.

kedi catkediye
to the cat

banka bankbankaya
to the bank

baba fatherbabaya
to the father

The buffer letter -N- with Ownership Condition

Buffer letter -N- is always used for the Ownership condition [genitive] -in -ın -ün -un

It becomes -nin -nın -nün -nun for "owners" that end in a vowel.

Bare nouns in the ownership condition do not use -Y- as a buffer letter.

Turkish Buffer letter -N- Ownership Condition
Bare NounExtended Noun
of the brıdge, the brıdge'sköprüsünün
of his brıdge, his brıdge's
of the table, the table'smasalarının
of their table, their table's
of the window, the window'spenceresinin
of its window, its window's
of the traveller, the traveller'syolcularının
of theır traveller, the traveller's

Buffer letter -N- is used with nouns that have already been suffixed.

This shift of buffer letter y to n enables the listener to discern that the word is already agglutinated (extended) by previous suffixes.

Buffer letter -N is used on ALREADY SUFFIXED NOUNS

kedisi his/her catkedisini
his/her cat
[3rd person possessed object]

bankası his/her bankbankasını
his/her bank
[3rd person possessed object]

kedileri their catkedilerine
to their cat
[3rd person plural possessed motion toward]

The Buffer Letter -N- with -da/-de and -dan/-den

The suffixes -de and -dan are added directly to basic nouns

kedi catkedide
on the cat

from the cat

kapı doorkapıda
on / at the door

from the door

They become -nda/-nde and -ndan/-nden when added to ALREADY SUFFIXED nouns.

This is to separate them from the extended word:

kedisi his/her catkedisinde
on his/her cat

from his/her cat

kapısı his/her doorkapısında
on/at his/her door

from his/her door

Buffer Letter -n used with Suffixes of Location -nde -nda and Movement -nden -ndan

‑de and ‑den require a buffer ‑n‑ when added to an extended word.

Simple NounExtended Noun
from the bankbankasından
from his bank
in/at the bankbankasında
in/at his bank
from the banksbankalarından
from their bank(s)
from the house/from homeevinden
from his house/home
in the housesevlerinde
in their houses