Most oceanic flows are nearly confined to 2D surfaces, called neutral surfaces, in which no work is done against gravity. Unfortunately, non-linearity in the density of seawater (a function of heat, salt, and pressure) and subtle differences in how heat and salt are distributed prevents the existence of neutral surfaces as well-defined 2D surfaces. Instead, a water parcel moving neutrally---never feeling gravity---and mixing as it goes around the ocean may return to its starting point above or below where it began. Neutral surfaces possess spirals. Nonetheless, the vertical pitch of these spirals is small, so it is useful to build well-defined 2D surfaces that closely approximate neutral surfaces. These are often an iso-surface of a 3D variable, such as Neutral Density (Jackett and McDougall, 1997) or Orthobaric Density (de Szoeke et al., 2000).