  Besides graphite and diamond, the solid allotropes of carbon in sp2 and sp3
hybridization, the possible existence of a third allotrope based on the
sp-carbon linear chain, the Carbyne, has stimulated researchers for a long
time. The advent of fullerenes, nanotubes and graphene has opened new
opportunities and nurtured the interest in novel carbon allotropes including
linear structures. The efforts made in this direction produced a number of
interesting sp-hybridized carbon molecules and nanostructures in the form of
carbon-atom wires. We here discuss some of the new perspectives opened by the
recent advancements in the research on sp-carbon systems.
