In words like

gönül ,-nlü
1. heart; mind.
2. inclination, desire, willingness.

boyun ,-ynu
1. neck.
2. cervix.
3. geog. pass, defile.
4. responsibility.

koyun ,-ynu
1. bosom, breast,
2. arms, embrace.

in which forms of the paradigm does the stem vowel disappear? In all cases sg and pl? What if a possessive suffix is added, does the vowel go or stay?

The last vowel in the stem in fact being concealed temporarly , it is disappearing from sight but not for good. Since that vowel is put back when we use 3.person plural as in example below ;

boyun --> neck

boynum --> my neck

boynun ---> your neck

boynu ----> his/her/its neck

boynumuz --> our neck

boyunları ----> their neck

-----------------------------------------------------------------------------------

boynumda --> in[on] my neck           boyunlarında --> in their neck

boynumdan --> from my neck           boyunlarından  --> from their neck

boynumu ---> accusative case         boyunlarını ----> accusative case

boynuma ---> dative case               boyunlarına ---> dative case

As you see in all cases , the last vowel in the stem disappears temporarly and comes back when we use with 3.Plural person [Onlar]

Ok, clear. But what if there was no possessive suffix but the case ending was attached straight to the stem?

NOM gönül

ACC gönl|ü

DAT gönl|e

LOC gönül|de

ABL gönül|den

GEN gönül|ün,

is it?

And it looks to me like the form given in the dictionary

gönül ,-nlü <----- here
1. heart; mind.
2. inclination, desire, willingness.

is the accusative.

If the case ending attached straight to the stem as in your example would be as follows;

NOM gönül 

ACC gönlü  ----------> Gönlü günahlardan temizlemek zor iş

DAT gönüle   -----> ´ Orhan Gazi´nin nasihatı 600 yıl boyunca gönülden gönüle ulaştı. ´  

LOC gönülde

ABL gönülden       

GEN gönlün

Thank you, tunci. Now there is a model here for everyone to follow. Trying to figure out the ruling on my own has cost me a good headache. The forms look so similar that you sometimes need to see them in a sentence before you can be sure. They even get mixed with other stems: boyun and boy with certain set of grammatical markers look so much the same sometimes in my eyes...

Learners of Turkish have been coddled with too much regularity I guess.

Good example, If we close our eyes and pick up the word "boy" from its context with genitive case attached , we would be left clueless about the meaning, such as ;

possible meanings of boyun ;

1.boyun --> Could be nominal case of the word " neck "

2.boyun --> Could be genitive case of "tribe"

   Boyun genel özelliği cesur, mert ve savaşcı olmasıydı.

   Warrior, redblooded and braveness was the general characteristic of the tribe.

3. boyun ---> Could be genitive case of the word "hight", "tall". [your hight ]

   Boyun kaç ?

   How tall are you ? [literally "how much is your tall ?]

In such stuations, if the words like "gönül, boyun, alın, ağız, kalp etc." takes a wovel, the second wovel from the stem is dropped. Examples:

boyun: boynu, boyna

alın: alnı, alna

ağız: ağzı, ağza

gönül: gönlü, gönle

kalp: kalbi, kalbe

It doesn´t matter wherher it is accusative or dative, the rule is the same as I explained above. But you should know the specific words which obey this rule.

Why does it look like words of this type are so often names of body parts?

It seems that you two disagree about the dative form. Google gave hits for both

           gönülden gönüle AND

           gönülden gönle

but the previous were much much more.

What I have explained in my previous post is the garmmatical rule, but sometimes in some cases -as you have mentioned here one of them- you can witness the different usage and they are exceptions.

The important thing here, the rule in my explanation is very easy to learn. The such words are so less in number and if you learn them and the rule I explained above you will not face any difficulty for using them.

As Metehan Hoca said, there are exceptional usages as in our case [dative case] we witness both ways and I ve gone for keeping the vowel. And in this particular case there is another subrule which is ;

These sort of words , if they are in reduplication form [ikileme] the vowel does not drop.Such as ;

Gönül gönüle

Gönülden gönüle

Ağız ağıza

Ağızdan ağıza

It´s understandable. Dropping the stem vowel in reduplicated forms would break the alliteration, the power of repeating.

Thanks for the other view, metehan2001.

This grammar rule is called "orta hece ünlüsünün düşmesi" (= dropping of the vowel in the middle syllable): 

The vowel in the middle syllabel in a grammatical unit consisting of more than two syllabels often drops when this middle syllable has no consonant at the end.

gö-nül

gö-nü-lü (the midde syllable has no consonant at the end) => (gön-lü gönlü

gö-nül-den (the middle syllabel has a consonant at the end) => gönülden

sa-bır

sa-bı-ret-mek => sab-ret-mek) sabretmek

sa-bı-re-den => (sab-re-den)  sabreden

sa-bır-dan => sabırdan

ka-hır

ka-hı-rol-mak => (kah-rol-mak) kahrolmak

ka-hır-lar => kahırlar

hü-zün

hü-zü-nü-ne => (hüz-nü-ne) hüznüne

hü-zün-de => hüzünde

ö-mür

ö-mü-rü => (öm-rü ömrü

ö-mür-den => ömürden

A new approach, scalpel.

It looks like a phonetic law which at a certain time has affected all stems of the type

              (C)VCVC-

What is notable in this group is that the stems are from different origins. The law has come like a wind and blown over all these stems no matter if they are own or borrowed.

There is a tendency for two-syllable words in all languages I guess. This shows in children´s language. Children sometimes even prolong one-syllable words to give them the ideal amount of syllables. I don´t know if this shows in Turkish.

Now the question is if there are stems of the type (C)VCVC- which do not follow this law. My vocabulary is too limited to look for them. I mean if it really was a phonetic law at some time all exceptions should have an explanation (like the word being younger for instance).

The only example that comes to my mind is uzun. Why uzun: uzunu but hüzün: hüznü? The etymological dictionary says the old stem is *uzı-, the second syllable was long until 1300 or so and there is actually and old affix in the end of uzun. Maybe this is the reason.

I wrote my original question for practical reasons  -  I felt I couldn´t derive the words  -  but it has turned out to be an interesting one. Now it seems that they are more a rule than an exception or am I wrong again?

Well,

Set the Arabic loans aside. Turkic words are all old words and very specifically are used for body parts so it´s a long development over years.

Alın - Aln-ı

Omuz - Omz-u

Karın - Karn-ı

Ağız - Ağz-ı

Burun - Burn-u

Boyun - Boyn-u

Koyun - Koyn-u

etc.

Why should I set them aside if they follow the same pattern? Speakers hardly make these distinctions.

I have understood phonetic laws  -  if this is one, it´s my guess  - work in a language for some limited time and go. That´s why I suggested it happened while the Arabic loans were already there. A phonetic law doesn´t ask where the word came from, it looks for certain phonetic combinations and operates in them like a machine.

You don´t seem to get my point.

They are different developments. Arabic loans come from Arabic without vowels in between. We add them according to Turkish phonetics. On the other hand Turkish ones drop their vowels as a result of some historic development of some specific class of words.

They come from different ends and they meet in the middle.

Yes. I really missed it. That the Arabic loans didn´t have the vowel in the first place and there must have been more steps in their development.

But my point is that phonetic changes don´t happen to certain groups of words, they happen to certain queues of phonemes.

Whatever the order of events has been a phonetic law has come and swallowed them all. Not one by one or group by group.

To the contrary of what some people think on this board, the rule is not limited with the old Turkish words for "body parts", nor with some of the exact Arabic origin words in Turkish..

And despite what they claim, koyun is not a body part. 

koyun - koynu

kayın, oğul, çığır are not body parts either but they follow the rule:

kayın - kaynı / kaynata, kaynana

oğul - oğlu

çığır - çığrı

Some of the passive forms of the verbs formed with factitive -r- follow the rule:

ça-ğı-rıl-mak => çağ-rıl-mak

bu-yu-rul-mak => buy-rul-mak

çe-vi-ril-mek => çev-ril-mek

ka-vu-rul-mak => kav-rul-mak

Derivatives of such words follow the rule as well:

ça-ğı-rış => çağ-rış

bu-yu-ruk => buy-ruk

çe-vi-rik => çev-rik

kav-vu-ruk => kav-ruk

Right scalpel. But it´s not a rule right? They are all old words and mostly the body parts (because most of them are old words). And they are the sound changes developed over a long period (I am talking about Turkish words). They are exceptions to be memorized and given in dictionary as such (usually where you see karın you also see karnı for example).

On second thoughts what I suggested about (C)VCVC stems can´t be true. From my lacking vocabulary I find plenty of (C)VCVC nouns which don´t lose any vowels but keep them tight. Such as okul, bedel, keder, kadın, bahar, besin, onur, ışık, atık, katır, umur, iman, beyaz. So much for that.

If it (orta hece ünlüsünün düşmesi) was not a rule how would you expain the missing i- in the following examples when idi, imiş, ise, iken, ile  lose their independence and become suffixes?

ben-im i-di => be-ni-mi-di => be-nim-di

benim imiş => benimmiş

benim ise => benimse

benim iken => benimken

benim ile => benimle

After all it is a phonetic event that can be seen here and there, and I don´t understand why some are insisted to imprison the rule only into the group of names for body parts.. 

***

There also can be two disappearing vowels in one word:

alınım idi => alnımdı

sıyırık imiş => sıyrıkmış

***

By the way here are some more words of the same type and they are not body parts at all:

ayrı, ayrık, ayrım, sıyrılmak, sıyrık, savrulmak, savruk, kavrulmak, kavruk, çağrı, çağrış, kıvrım, kıvrık, bağrış, buyruk, evrim, çevrim, devrim, devrik, etc..

Some of these words are very new and apparently they didn´t need a long period to be developed  

What is a rule? How can you state it for vowel dropping? Does it always apply?

Oh yes they are accumulation of long periods.

Your examples are dictionary entries (derivational forms) and they are well documented. I was talking about inflectional forms of some words (i.e. "alın" is a dictionary entry but sometimes it becomes "alnı" for example) and they are mostly seen for body parts. (I know you are after the rules but I am not stating a rule it´s only an observation.)

Another observation:

Your examples are mostly of the the form (using regular expressions)

*[ğvy]?r* and ? is dropped

Here is an exception for it:

doğurmak -> doğurulmak

alın and omuz for example are not of that form:

alın -> alnım

omuz -> omzum

This kind of dropping is surely a development over long periods.