  Phase noise and frequency (in)stability both describe the fluctuation of
stable periodic signals, from somewhat different standpoints. Frequency is
unique compared to other domains of metrology, in that its fluctuations of
interest span at least 14 orders of magnitude, from $10^{-4}$ in a mechanical
watch to $10^{-18}$ in atomic clocks. The frequency span of interest is some
12-15 orders of magnitude, from $\mu$Hz to GHz Fourier frequency for phase
noise, while the time span over which the fluctuations occur ranges from
sub-$\mu$s to years integration time for variances. Because this domain is
ubiquitous in science and technology, a common language and tools suitable to
the variety mentioned are a challenge.
  This article is at once (1) a tutorial, (2) a review covering the most
important facts about phase noise, frequency noise and two-sample (Allan and
Allan-like) variances, and (3) a user guide to "Enrico's Chart of Phase Noise
and Two-Sample Variances." In turn, the Chart is a reference card collecting
the most useful concepts, formulas and plots in a single A4/A-size sheet,
intended to be a staple on the desk of whoever works with these topics. The
Chart is available under Creative Commons 4.0 CC-BY-NC-ND license from Zenodo,
DOI 10.5281/zenodo.4399218. A wealth of auxiliary material is available for
free on the Enrico's home page http://rubiola.org.
