In English, all that separates Dan from a Dane and Jan from Jane is a shorter vowel sound. Similarly, in German, a Beet (flower bed) becomes a Bett (bed) as the vowel shifts from long to short.
Double vowels, referring to two consecutive identical vowels, always produce a long vowel sound. This spelling developed as an intuitive way to mark a long vowel sound. Examples include: Aal (eel), Boot (boat), See (lake), and Tee (tea). Notably, this concept also exists in English, as illustrated by the word pairs bleed vs. bled, feed vs. fed, and root vs. rot.
Another common marker of a long vowel is the letter h. This spelling dates back to earlier forms of German, where many words featured an audible h sound after a vowel, like the now-outdated word Zahan (tooth). Over time, the h sound was lost, but the spelling remained to indicate the long vowel. Modern examples include Bahn (train), Huhn (chicken), Lohn (wage), and Zahn (tooth).