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I came across いただきとう, as in
「[納めさせていただきとうございます](https://blog.goo.ne.jp/andante625j/e/f5590b2960970d762546902004e311ae)」
or
「[軽く説明させていただきとうございます](https://na.finalfantasyxiv.com/lodestone/character/2165543/blog/1865877)」.
My research led me to [this
question](https://japanese.stackexchange.com/q/25141) where I learned とう is
likely just an ウ音便 of たい. This seems a fairly rare locution these days.

**#1.** Why does いただきとう always seem to collocate with ござる/ございます? Is it because
it is a fossil from a different age?

**#2.** This comment piqued my curiosity:

> You have actually been using this 「とう」 in saying 「ありがとう」 ever since you
> started learning the language. – l'électeur Jun 19 '15 at 7:08

It instantly makes sense to me: ありがとう―ありがたい. But then I recall often seeing
ありがとう rendered in kanji: 有難う{ありがとう} or 有り{あり}難う{がとう}, especially in verbiage
used by older people in situations that call for slightly formal language. If
とう is a monolithic morphological unit, why does a break occur in the kanji
representation between と and う?
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grammar syntax morphology
いただきとう and ありがとう
Of your two questions,
@[istrasci's](https://japanese.stackexchange.com/users/78/) suggested link in
the comments should address your first question, about collocation with ございます.

The second question is more about spelling conventions and morphology. The
linked post doesn't quite get into this, so I'll address that here.

# If とう [as an adverbial ending] is a monolithic morphological unit, why does
a break occur in the kanji representation between と and う?

Your question contains within it the seeds of the ansswer.

The key is that ～とう is not entirely a monolithic morphological unit. It
_looks_ like one, but it is not treated as one.

## Origins of the ～とう ending

As you describe, the ～とう ending is _" an ウ音便 of たい"_. This ウ音便【おんびん】 is
basically a matter of dropping the //k// from adverbial ending ～く, and then
adjusting the corresponding vowel + vowel in ways that mirror historical sound
shifts.

_**Note:** The elision (removal) of that medial //k// is also what gives us
the modern_ ～い _attributive and terminal / predicative endings for so-called_
-i _adjectives: in Classical Japanese and earlier, the attributive form for
these adjectives ended in_ ～き _instead._

### Vowel fusion

The vowels fuse as follows:

  * あ + う → おう  
Examples:

    * あらがたい → ありがたく → ありがたう → ありがとう
    * おめでたい → おめでたく → おめでたう → おめでとう
  * い + う → ゆう  
Examples:

    * よろしい → よろしく → よろしう → よろしゅう
    * おおきい → おおきく → おおきう → おおきゅう
  * う + う → うう  
Examples:

    * にくい → にくく → にくう
    * やすい → やすく → やすう
  * え + う → よう  
I can't find any adjective examples involving the ～え → ～う shift, simply
because there are so few adjectives ending in _-ei_ (I can only find
執念【しゅうね】い, "stubborn, persistent"). However, there are other historical
examples we can show:

    * 今日 was けふ (//kepu//) in Old Japanese. That medial //p// lenited (softened) first into //f// producing //kefu// (technically //keɸu//), then into nothing, leaving けう (//keu//). That then shifted into modern きょう.
    * 食べよう ("let's eat") was formerly たべむ. That medial //m// lenited, producing たべう. In roughly the Muromachi period, this was pronounced more like たびょう, until the volitional ending was re-analyzed as suffix ～よう, producing たべよう.
  * お + う → おう  
Examples:

    * おおい → おおく → おおう
    * すごい → すごく → すごう

### Morphology and okurigana

Broadly speaking, okurigana are added onto the end of a kana spelling to
indicate that portion of the reading that varies with different conjugations.

Sometimes this reflects changes that _used to happen_ in Classical Japanese
and earlier, but that don't happen anymore. Two whole classes of verbs reflect
this: the so-called 上【かみ】一段【いちだん】活用【かつよう】動詞【どうし】 (literally "upper monograde
conjugation verbs"; _kami_ or "upper" refers to the い vowel that appears on
the end of the verb stem, and い is considered "higher" than え, the end of the
other kind of monograde verb stem), and the 下【かみ】一段【いちだん】活用【かつよう】動詞【どうし】
(literally "lower monograde conjugation verbs", where _shimo_ or "lower"
refers to the え vowel that appears on the end of the verb stem, and え is
considered "lower" than い).

The _ichidan_ or "monograde" part refers to the fact that there is only one
verb stem for all of the conjugations. Let's look at two verbs as examples:

Form | _Kami_ verbs with "i" stems | _Shimo_ verbs with "e" stems  
---|---|---  
終止形【しゅうしけい】  
Terminal / predicative  
(dictionary form) | [起]{●}[き]{●}る  
 **oki** ru | [食]{●}[べ]{●}る  
 **tabe** ru  
未然形【みぜんけい】  
Incomplete / irrealis  
(for negatives, etc.) | [起]{●}[き]{●}～  
 **oki** - | [食]{●}[べ]{●}～  
 **tabe** -  
連用形【れんようけい】  
Continuative / infinitive  
(for ～ます, etc.) | [起]{●}[き]{●}～  
 **oki** - | [食]{●}[べ]{●}～  
 **tabe** -  
連体形【れんたいけい】  
Attributive  
(for modifying nouns) | [起]{●}[き]{●}る  
 **oki** ru | [食]{●}[べ]{●}る  
 **tabe** ru  
已然形【いぜんけい】  
・仮定形【かていけい】  
Realis / hypothetical  
(for conditionals, etc.) | [起]{●}[き]{●}れ～  
 **oki** re- | [食]{●}[べ]{●}れ～  
 **tabe** re-  
命令形【めいれいけい】  
Imperative  
(for making commands) | [起]{●}[き]{●}ろ・[起]{●}[き]{●}よ  
 **oki** ro / **oki** yo | [食]{●}[べ]{●}ろ・[食]{●}[べ]{●}よ  
 **tabe** ro / **tabe** yo  
  
As you can see above, the verb stem portion remains the same for all verb
forms.

However, in Classical Japanese and older stages of the language, both of these
conjugation paradigms used to be 二段【にだん】 or "bigrade", and that last kana on
the verb stem used to _also_ change. In these older conjugation paradigms, the
_nidan_ or "bigrade" part of the name refers to the alternation between either
"i" and "u", or "e" and "u", as seen here:

Form | _Kami_ verbs with "i" stems | _Shimo_ verbs with "e" stems  
---|---|---  
終止形【しゅうしけい】  
Terminal / predicative  
(dictionary form) | [起]{●}[く]{●}  
 **ok** u | [食]{●}[ぶ]{●}  
 **tab** u  
未然形【みぜんけい】  
Incomplete / irrealis  
(for negatives, etc.) | [起]{●}[き]{●}～  
 **ok** i- | [食]{●}[べ]{●}～  
 **tab** e-  
連用形【れんようけい】  
Continuative / infinitive  
(for ～ます, etc.) | [起]{●}[き]{●}～  
 **ok** i- | [食]{●}[べ]{●}～  
 **tab** e-  
連体形【れんたいけい】  
Attributive  
(for modifying nouns) | [起]{●}[く]{●}る  
 **ok** uru | [食]{●}[ぶ]{●}る  
 **tab** uru  
已然形【いぜんけい】  
・仮定形【かていけい】  
Realis / hypothetical  
(for conditionals, etc.) | [起]{●}[く]{●}れ～  
 **ok** ure- | [食]{●}[ぶ]{●}れ～  
 **tab** ure-  
命令形【めいれいけい】  
Imperative  
(for making commands) | [起]{●}[き]{●}よ  
 **ok** iyo | [食]{●}[べ]{●}よ  
 **tab** eyo  
  
The changeability of that last kana in the stem is why the き and the べ are
still included in the okurigana for these verbs -- even though that last kana
of the verb stem no longer changes in modern usage.

### Looking back at that ～とう ending

The treatment of okurigana for the ～とう ending has some parallels to the
treatment of okurigana for _ichidan_ verbs. Since the た portion of the ～たい
ending doesn't _functionally_ change into a different kana, that た is treated
as part of the unchanging stem of the adjective.

Granted, the sound changes from _ta_ to _to_. However, this only reflects
[euphony](https://en.wikipedia.org/wiki/Japanese_grammar#Euphonic_changes_\(%E9%9F%B3%E4%BE%BF_onbin\))
through the fusing of adjacent vowels. That's what I mean when I say that this
isn't a _functional_ change -- the sound shift does not indicate a change in
conjugation forms.

Consequently, this ～とう ending is still treated as if it were ～た (part of the
stem) + う (adverbial suffix) for purposes of figuring out the okurigana, and
thus this ～とう is morphologically treated as two units instead of one.
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answer