  Stellar triples with massive stellar components are common, and can lead to
sequential binary black-hole mergers. Here, we outline the evolution towards
these sequential mergers, and explore these events in the context of
gravitational-wave astronomy and the pair-instability mass gap. We find that
binary black-hole mergers in the pair-instability mass gap can be of triple
origin and therefore are not exclusively formed in dense dynamical
environments. We discuss the sequential merger scenario in the context of the
most massive gravitational-wave sources detected to date: GW170729 and
GW190521. We propose that the progenitor of GW170729 is a low-metallicity field
triple. We support the premise that GW190521 could not have been formed in the
field. We conclude that triple stellar evolution is fundamental in the
understanding of gravitational-wave sources, and likely, other energetic
transientsas well.
