「じ」 or 「ぢ」？ 「ず」 or 「づ」？
When you learned hiragana, did you think about the difference between 「じ」 and 「ぢ」, and between 「ず」 and 「づ」, as each pair of hiragana has the same pronunciation?
The pronunciation of 「じ」 and 「ぢ」 is ’zi’ in modern Japanese ("gee" in English pronunciation). But in the past「ぢ」was pronounced as ‘di’.
On the same note, the pronunciation of 「ず」 and 「づ」is ’zu’ in modern Japanese but 「づ」used to be pronounced as ‘du’.
After many years, in all but a few dialects (in southern part of Japan), 「ず」 merged with「づ」while「じ」merged with「ぢ」.
After the spelling system has reformed, in most of the following 2 cases, 「じ」 and 「ぢ」were retained:
1. Words with a repeated sound: 続（つづ）きます – continue; 少（すこ）しづつ – little by little
2. In compounds with "double dots": 鼻血（はなぢ）– nosebleed; 入（い）れ知恵（ぢえ）– put an idea into
In all other cases, 「ぢ」was replaced with 「じ」 and 「づ」 with 「ず」. There are still some exceptions, especially for old words. However, it's not necessary to remember all of them as Japanese use 「じ」and 「ず」in most cases in currently :)