  We present an interesting monogamy equation for $(2 \otimes 2 \otimes
n)$-dimensional pure states, by which a quantity is found to characterize the
tripartite entanglement with the GHZ type and W typeentanglements as a whole.
In particular, we, for the first time, reveals that for any quantum state of a
pair of qubits, the difference between the two remarkable entanglement
measures, concurrence and negativity, characterizes the W type entanglement of
tripartite pure states with the two-qubit state as reduced density.
