LANGELIER INDEX:  Also called the Saturation Index, Mr. Langelier devised a system to determine water balance by assigning values to levels of pH, Total Alkalinity, Calcium Hardness and water Temperature. When all parameters are in balance, the water will neither be corrosive or scaling. The formula is "SI = pH + TF + CF + AF - 12.1."

LATERALS:  Elongated, capped plastic nipples at the bottom of a sand filter which are slotted to allow for water passage while keeping the sand in the filter tank.

LIGAND: An atom, ion, or molecule that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as a Lewis base).

LOAD:  An electric device which consumes energy, placing a load on the source.

LOW WATER SUCTION:  An influent fitting, typically low on the wall in the deep end of a vinyl liner pool. A cheaper alternative to a main drain.