I tried internal and external DTD's both with a name different than the root element name , it still Works!.
Its about the value of "Name" in the tag below -
<DOCTYPE Name SYSTEM "Couple.dtd">
What's the use of this "Name" then?
1)Internal DTD
<!DOCTYPE SampleDocType[
<!ELEMENT Neighbours (Couple, Couple) >
<!ELEMENT Couple (Name, Name) >
<!ELEMENT Name ( Vorname, Nachname, Geschlecht) >
<!ELEMENT Vorname (#PCDATA) >
<!ELEMENT Nachname (#PCDATA) >
<!ELEMENT Geschlecht (#PCDATA) >
]>
<Couple>
<Name>
<Vorname>Marie</Vorname>
<Nachname>Müller</Nachname>
<Geschlecht>=</Geschlecht>
</Name>
<Name>
<Vorname>Klaus</Vorname>
<Nachname>Müller</Nachname>
<Geschlecht>♂</Geschlecht>
</Name>
</Couple>
2)External DTD
<!DOCTYPE SampleDocType SYSTEM "Couple.dtd" >
<Couple>
<Name>
<Vorname>Marie</Vorname>
<Nachname>Müller</Nachname>
<Geschlecht>=</Geschlecht>
</Name>
<Name>
<Vorname>Klaus</Vorname>
<Nachname>Müller</Nachname>
<Geschlecht>♂</Geschlecht>
</Name>
</Couple>
external DTD file
<!ELEMENT Neighbours ( Couple, Couple) >
<!ELEMENT Couple (Name, Name) >
<!ELEMENT Name ( Vorname, Nachname, Geschlecht) >
<!ELEMENT Vorname (#PCDATA) >
<!ELEMENT Nachname (#PCDATA) >
<!ELEMENT Geschlecht (#PCDATA) >
Note - I am using XMLtools plugin in Notepad++ for validation.