German and Dutch are both languages of the West Germanic language family. They share many similarities in vocabulary and grammar. However, their phonology is quite different. This can be observed in the quality of the low central and back vowels.
Both languages have short and long vowels. In Dutch the word raam (window) has a long vowel and appel (apple) has a short vowel. Likewise, the German word Sahne (cream) has a long vowel and Nacht (night) has a short vowel. However, the features of the vowels in the words are different in the two languages.
The vowel in the word raam is a low central vowel, and in the word appel it is low back. The quality of the two vowels is different because the long vowel is low central and the short vowel is low back. This is not the same in German.
The word Sahne has a long low central vowel, and the word Nacht has a low central vowel. In both words the vowel quality is the same. The difference in the two vowels is quantitative. The duration of the long vowel is greater than the duration of the short one.
The phonology of German and Dutch is not identical. The two languages both have short and long vowels, but the low central/low back vowels are different. In Dutch the short and long vowels differ in vowel quality, but in German they differ in vowel quantity. The difference in the vowels of German is thus quantitative, but in Dutch it is qualitative.