Append a Lists Contents to another List C#

I have the following:

1. A main List called GlobalStrings
2. Another List called localStrings

In a loop for example:

List<string> GlobalStrings = new List<string>(); 
List<string> localStrings = new List<string>();
for(x=1;x<10;x++)
{
    localStrings.Add("some value"); 
    localStrings.Add("some value");
}
// Want to append localStrings to GlobalStrings as easily as possible