The extreme precision of optical atomic clocks has led to an anticipated redefinition of the second by the International System of Units. Furthermore, accuracies pushing the boundary of 1 part in 10
18
and beyond will enable new applications, such as in geodesy and tests of fundamental physics. The
1
S
0
to
3
D
1
optical transition in
176
Lu
+
has exceptionally low sensitivity to external perturbations, making it suitable for practical clock implementations with inaccuracy at or below 10
−18
. Here, we perform high-accuracy comparisons between two
176
Lu
+
references using correlation spectroscopy. A comparison at different magnetic fields is used to obtain a quadratic Zeeman coefficient of −4.89264(88) Hz/mT for the reference frequency. With a subsequent comparison at low field, we demonstrate agreement at the low 10
−18
level, statistically limited by the averaging time of 42 hours. The evaluated uncertainty in the frequency difference is 9 × 10
−19
and the lowest reported in comparing independent optical references.