  We show that $n$-tangle, the generalization of the 3-tangle to even $n$
qubits, is the square of the SLOCC polynomial invariant of degree 2. We find
that the $n$-tangle is not the residual entanglement for any even $n\geq 4$\
qubits. We give a necessary and sufficient condition for the vanishing of the
concurrence $C_{1(2...n)}$. The condition implies that the concurrence $%
C_{1(2...n)}$ is always positive for any entangled states while the $n$%
-tangle vanishes for some entangled states. We argue that for even $n$\ qubits,
the concurrence $C_{1(2...n)}$\ is equal to or greater than the $n$% -tangle.
Further,\ we reveal that the residual entanglement is a partial measure for
product states of any $n$ qubits while the $n$-tangle is multiplicative for
some product states.
