  We summarize the four tests of general relativity carried out using the ten
binary black hole signals detected by Advanced LIGO and Advanced Virgo and
included in their first catalog, GWTC-1. These events provide unprecedented
opportunities for testing the predictions of general relativity for the
gravitational waveforms from these highly dynamical, strong-field events. The
first two tests check the consistency of the residuals with noise and the
consistency of the low- and high-frequency parts of the signal. The other two
tests check that parameterized deviations in the waveform model---including in
the post-Newtonian coefficients---are consistent with zero, and that the
propagation of the waves is nondispersive. These tests reveal no evidence for
deviations from general relativity, and the combined constraints improve
previous results by factors of up to 2.5. We also check that the binary black
hole signals observed by all three detectors do not give stronger constraints
on alternative polarizations than those obtained from GW170817.
