Mass is not placed on a scale.
We infer it from the way spacetime rang, inside a model that could have been wrong.
01 · Chirp mass
During inspiral, the rate at which the frequency rises is set mostly by a combination of the two masses called the chirp mass. It is the braided weight that sets how fast the song rises, and the best-measured mass-like quantity in a typical binary black-hole signal.
02 · Amplitude and distance
Strain amplitude falls with luminosity distance. A louder nearby event can mimic a more massive distant one unless the frequency evolution is read at the same time. Distance and mass talk to each other.
03 · Mass ratio and spin
Which hole is heavier, and how they spin, is harder. Higher harmonics, precession and the merger–ringdown shape help separate the two component masses. These are more model-dependent than chirp mass. Different waveform families can shift the medians.
04 · Frames
The expanding universe stretches the wave on the way here, so the masses we hear are heavier than the masses at the source. Detector-frame masses are redshifted: mdet = (1 + z) msource. Catalogs usually quote source-frame masses, after undoing that stretch. Cosmology enters when redshift is inferred from distance. That is a further model.
Bayesian parameter estimation against general-relativistic waveform models (for GWTC-4.0: among them IMRPhenomX families, NRSur7dq4, SEOBNRv5PHM, and mixed combinations). 90% credible intervals are the catalog convention.