giyeok pronunciation rules?

[Hokva]

Hello everyone!

I read that ㄱ is pronounced either like an English "g" or "k" and that ㄷ is pronounced either like a "t" or "d."

Does anyone know if the production of the sound of each letter actually differs from word to word or is it simply that the letter sounds different when preceded or followed by certain letters? In other words, are the mouth shape, tongue position and flow of air the same when uttering a giyeok "k" sound and a "g" sound?

If the pronunciations do differ, are there any rules on when a letter is pronounced what way?

[Rounin]

From http://en.wikipedia.org/wiki/Korean_phonology :

|p, t, tɕ, k| become voiced [b, d, dʑ, g] between voiced sounds.

[Hokva]

Thanks, Rounin! =)