How do 自他 triplets of related verbs work?


I thought this had been asked before (possibly by me), but I couldn't find it.
I'm wondering why for certain verbs/words, instead of just a `自他` pair, there
is a triplet (or possibly more) where two of the verbs are one type (`自` or
`他`) and the third is the other type. Some examples:

>   * 他 → 縮【ちぢ】める ： 自 → 縮【ちぢ】まる ： 縮【ちぢ】む → both?
>     * see @snailboat comment below - dictionary confirms. Exs.
>       * 自 → 寿命が縮む ： 他 → 「ちぢめるの文語形」in dictionary def.
>   * 他 → 繋【つな】ぐ ・ 繋【つな】げる ： 自 → 繋【つな】がる
>   * 他 → 緩【ゆる】める ： 自 → 緩【ゆる】まる ： 緩【ゆる】む → both? (same as above)
>   * 他 → 滅【ほろ】ぶ ・ 滅【ほろ】びる ： 自 → 滅【ほろ】ぼす
>   * [他 → 含【ふく】む ・ 含【ふく】める ： 自 →
> 含【ふく】まる](https://japanese.stackexchange.com/q/14103/78)
>

What's the deal with these triplets? Why are there two accepted verbs of one
form for the same meaning? Are they somehow different? Is one of the two a
classic/obsolete form like [I talked about
here](https://japanese.stackexchange.com/q/5506/78)? Does one of the two
somehow rise to dominance over the other? I admit that I encounter `繋【つな】ぐ` a
lot less than `繋【つな】げる`, but `縮【ちぢ】む` seems just as frequent as `縮【ちぢ】まる` to
me.

Any other examples would be great to list too.



[code]

    What's the deal with these triplets?
    Why are there two accepted verbs of one form for the same meaning? 
    Are they somehow different? 
    
[/code]

Yes, their usage is slightly different. A web article below well answers your
question:

**Coexistent transitive verbs : "Tsunagu" and
"Tsunageru"<http://ci.nii.ac.jp/naid/110004672022>**

For '繋げる/繋ぐ' case:

'繋ぐ' refers to connecting something that are meant to be connected, or are
naturally connected, or are desired to be connected.

切れたひもをつなぐ/鎖とロープをつないだ/多くの塔をつないだ高い城壁が (from the article）

On the other hand '繋げる' is to connect something that are not naturally
connected, so there should be some external force for them to be connected.

6台のテレビをつなげ、GT3を複数画面で同時プレイ可能に.../短いひもをつなげて長くする (from the article)

Sometimes it may be hard to determine whether the connected state is natural /
unnnatural: In such cases, either is OK. For example,

[code]

    ひもをつなげて長くする/ひもをつないで長くする
    
[/code]

usually both are accepted.

[code]

    Is one of the two a classic/obsolete form like I talked about here? 
    
[/code]

In your another question [Rare/Obsolete verb
forms](https://japanese.stackexchange.com/questions/5506/rare-obsolete-verb-
forms) for example, 'ほむべきお方' is not usually used in modern Japanese. Therefore
out of 'ほむ/ほめる', 'ほむ' is obsolete. But '繋ぐ/繋げる' are both frequently used
expressions. It depends on the word. As far as the examples you have mentioned
in your question here, all expressions are currently widely used.

[code]

    Does one of the two somehow rise to dominance over the other? 
    I admit that I encounter 繋つなぐ a lot less than 繋つなげる, 
    but 縮ちぢむ seems just as frequent as 縮ちぢまる to me.
    
[/code]

Maybe you are encountering a lot of naturally connected things than
unnaturally connected things. And you are seeing naturally shrunk/contracted
things as frequently as artificially shrunk/contracted things.

