た　ち　つ　て　と

In textbooks or other websites, you may see either ji or di for ぢ, and either zu, du or even dzu for づ.

There are two other Hiragana which have virtually the same sound, じ (ji) and ず (zu).

Historically ぢ / じ, and づ / ず, had distinct sounds, but in modern Japanese the sounds for these pairs of Hiragana are now indistinguishable in most Japanese dialects (though there still are some exceptions) and they now both sound closest to the English pronunciations of ji and zu.

In fact many words which were originally written with ぢ and づ Hiragana, have been replaced with the じ and ず Hiragana in modern Japanese. The results is that both ぢ and づ are characters that only exist now in a small sets of words.

The reason why we've chosen to use di and du on this website is to distinguish the characters from じ and ず, and because when you type Hiragana on a keyboard, the roman letters du converts to づ, and di converts to ぢ. The practice games work this way as well, so that when you progress to typing in Japanese, you don't need to change how you think.

So its generally useful to remember the romaji that way. In saying that, when you are reading or speaking Japanese words, it is important that you don't think of the romaji at all, and think just on the Japanese pronunciation of the word. This is especially important for other sets for Hiragana like 「ら、り、る、れ、ろ」where the Japanese pronunciation for the (r) sound is slightly different from the English pronunciation.

If you would like to hear the sounds of the characters, the following words are useful:

づ

続(つづ)く(く)「つづく」

ぢ

近(ちか)々(ぢか)「ちかぢか」

ず

静(しず)か(か)「しずか」

じ

信(しん)じ(じ)る(る)

There is a link to stackexchange as well which has some interesting further reading.

Really? I thought it meant di and du, all the sorces I have looked through it said that, every textbook, I went through it said di and du, I might have to look deeper into this, it could be an "another dialect" situation

Im also pretty sure that it means JI and Zu I always get wrong answers for that in the flashcards even tho It says in Kinda every textbook and Website its Ji and Zu

i'm pretty sure that  ぢ is meant to be ji, and づ is supposed to be zu.