  We answer frequently asked questions (FAQs) about the Hellings and Downs
correlation curve -- the "smoking-gun" signature that pulsar timing arrays
(PTAs) have detected gravitational waves (GWs). Many of these questions arise
from inadvertently applying intuition about the effects of GWs on LIGO-like
detectors to the case of pulsar timing, where not all of it applies. This is
because Earth-based detectors, like LIGO and Virgo, have arms that are short
(km scale) compared to the wavelengths of the GWs that they detect (approx
100-10,000 km). In contrast, PTAs respond to GWs whose wavelengths (tens of
light-years) are much shorter than their arms (a typical PTA pulsar is hundreds
to thousands of light-years from Earth). To demonstrate this, we calculate the
time delay induced by a passing GW along an Earth-pulsar baseline (a "one-arm,
one-way" detector) and compare it in the "short-arm" (LIGO-like) and "long-arm"
(PTA) limits. This provides qualitative and quantitative answers to many
questions about the Hellings and Downs curve. The resulting FAQ sheet should
help in understanding the "evidence for GWs" recently announced by several PTA
collaborations.
