what exactly is string length?

[mw]

https://run.dlang.io/is/B4jcno

---
import std;
import std.conv : text;


void main()
{
     char[6] s;
     s = "abc";
     writeln(s, s.length);  // abc6, ok it's the static array's 
length

     string t = text("head-", s, "-tail");
     writeln(t, t.length);  // head-abc-tail16, why?
}
---

Why the last output is 16 instead of 13, t's type is string here.