If you are studying modern Japanese, you are familiar with the phonetic kana writing system. What you might not be aware of, is that the orthography used today was put into use as recently as 1946. Before then, kana spelling was much more irregular (think of English spelling versus pronunciation), since the same spelling had been in use for centuries, and the pronunciation had changed over time.
Before the 1946 reforms, there were also two kana that are no longer in use today; ゐ/ヰ (wi) and ゑ/ヱ (we). They were in the same row as を/ヲ(wo), and for some time had been pronounced like i and o. In the 19th century, ゑ/ヱ was pronounced closer to ye, which is why the modern Japanese currency is spelled Yen in English, but is still pronounced en in Japanese.
Here are some examples of post-war spelling changes:
“to laugh”: わらふ (warafu) becomes わらう (warau)
“let’s laugh”: わらはう (warahau) わらおう (waraou)
“to say”: いふ (ifu) いう (iu)
“let’s say”: いはう (iahau) いおう (iou)
“today”: けふ (kefu) きょう (kyou)
“butterfly”: てふ (tefu) ちょう (chou)
“voice”: こゑ (kowe) こえ (koe)
For more information: https://en.wikipedia.org/wiki/Historical_kana_orthography