What does the use of a dash (instead of a character) to extend a sound mean?

Question

I came across the sentence 混乱する気持ちもよーくわかる in my manga. I have translated it, but the use of the dash (which was vertical in the actual vertical text) stumped me for a bit (I thought it was よう at first, not よお). I was under the impression that a dash like that is only used in katakana, and in hirigana they use the character of the sound they want to extend. But that's not the case here.

So what does it mean when this happens? Is is a special case or exception, or is there some rule?

By the way, I ended up with よーくわかる all together meaning "I know you..." (thanks, Google Translate, for being more useful than a dictionary for once), which seems to be right in this context (In this case, "I know you're feeling confused"). That's why I think it might be a special case. (Searching よお on it's own ended up with "trouble brought on by sins of forebears"...)

Answer 1

In Katakana, we use ー for some long vowels indeed. But words with it, like ユーロ are spelt this way!

However, in your case, there is no such word よーく、 ようく nor よおく. What this dash means is that the sound is lengthened. The word is just "よく". So, when the author wrote "よーくわかる" he meant "I reaaaaally understand".

That's it!

Answer 2

I understand, In Japanese, the long dash (ー) means the sound is lengthened, just as Axioplase said. Like this:

biiru
ビール (beer)

keeki
ケーキ (cake)
Sometimes, when writing in Romaji, (the English style of writing Japanese), the 'dash' is substituted with the letter and a small line over it. Like this:

Kēki

Bīru

Answer 3

Japanese call cellphones Kei Tai Denwa (literally portable phone) but they don't use the kanji for keitai 携帯, they don't use the hiragana, けいたい, they use katakana ケータイ, which uses a hyphen when clearly it should use ケイタイ.

It just makes it look cool and international to use it like that.