  It has become increasingly useful to answer questions in gravitational-wave
astronomy using transdimensional models where the number of free parameters can
be varied depending on the complexity required to fit the data. Given the
growing interest in transdimensional inference, we introduce a new package for
the Bayesian inference Library (Bilby) called tBilby. The tBilby package allows
users to set up transdimensional inference calculations using the existing
Bilby architecture with off-the-shelf nested samplers and/or Markov Chain Monte
Carlo algorithms. Transdimensional models are particularly helpful when we seek
to test theoretically uncertain predictions described by phenomenological
models. For example, bursts of gravitational waves can be modelled using a
superposition of N wavelets where N is itself a free parameter. Short pulses
are modelled with small values of N whereas longer, more complicated signals
are represented with a large number of wavelets stitched together. Other
transdimensional models have found use describing instrumental noise and the
population properties of gravitational-wave sources. We provide a few
demonstrations of tBilby, including fitting the gravitational-wave signal
GW150914 with a superposition of N sine-Gaussian wavelets. We outline our plans
to further develop the tbilby code suite for a broader range of
transdimensional problems.
