2D liquids are related to 2D gases. If the density of a 2D liquid is decreased, a 2D gas is formed. This was observed by scanning tunnelling microscopy under ultra-high vacuum (UHV) conditions for molecular adsorbates. 2D liquids are related to 2D solids. If the density of a 2D liquid is increased, the rotational degree of freedom is frozen and a 2D solid is created.