Mutually intelligible languages are those which allow speakers to understand each other in both written and oral contexts, without the necessity of studying or having much knowledge of the other language. This is what happens between native speakers of Danish, Swedish and Norwegian; it also is the case for speakers of Afrikaans and Dutch.
The Germanic languages were separated from the rest of the Indo-European languages by the evolution of its vowels and consonants, as well as intonation. In this sense, we’re referring to Afrikaans, German, Danish, Faroese, Frisian, English, Icelandic, Dutch (along with Flemish), Norwegian, and Swedish.
The Nordic languages (northern Germanic) began to differentiate themselves from the other Germanic languages around 200 AD. It was a language without a proper name, though until the 13th century in Sweden it was common to refer to the Danish language. Nordic shares many similarities with Old English, which it influenced heavily overall during the Viking period. Words such as fellow, husband, they, them, their, sky, window, live, and die come from the Nordic languages. Since the Medieval Era, the Nordic languages have received the same influences as the rest of the European languages: from the classic languages, from German, French and Italian. As of the middle of the 20th century all of these languages have been fundamentally influenced by English.
A characteristic trait of Danish and the rest of the Nordic languages is the presence of the enclitic definite article. While in English, German, French and other Romance languages the determination (definite or indefinite) occurs by placing an element beforehand, the Nordic languages add a desinence to the noun: the house, das Haus, la maison, la casa, huset (indefinite form: hus).