Difference between ぢ and じ & づ and ず?

Question:

I'm learning Hiragana and came across ぢ and じ which both mean 'ji' and づ and ず which both mean 'zu'. What's the difference? Is it a matter of choice between the characters when writing?

Answer:

1. ぢ and じ

As far as I know, ぢ is only used for nosebleed.... 鼻血はなぢ(You need to type: "hanadi") hana-ji

2. づ and ず

Most of the case, ず Usage of づ is limited.

つづく to be contined
つづり spelling
some more?

or, when the word is combined with anohter expression. For example,
おづる／めづる male crane/female crane ← つる
つらい hard　→ みづらい hard to see　
おこづかい（pocket money）← つかう
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