# External consistency vs linearizability

In Spanner, TrueTime & The CAP Theorem, Eric Brewer writes:

One subtle thing about Spanner is that it gets serializability from locks, but it gets external consistency (similar to linearizability) from TrueTime [emphasis added].

What is the definition of external consistency, and how does it differ from linearizability?

For any two transactions, $T_1$ and $T_2$ (even if on opposite sides of the globe): if $T_2$ starts to commit after $T_1$ finishes committing, then the timestamp for $T_2$ is greater than the timestamp for $T_1$.