Dilution

From Silvergrain Labs

Developer and other processing solution may be prepared or supplied in liquid concentrate form to reduce the bulk, to reduce the transportation cost and storage space. Concentrated solution is diluted before use.

In photography, dilution ratio is usually indicated by the part concentrate to the part water. For example, 1:3 means one part concentrate is mixed with 3 parts water. Eastman Kodak Company has used this notation, but this is different from standard notation in chemistry and most other parts of science, where 1:3 would mean one part concentrate to 2 parts water (to make 3 parts diluted solution). Therefore, the symbol "+" is often used in place of ":" to indicate this fact. The dilution of 1+3 means one part concentrate is mixed with 3 parts water. Ilford has used this notation. This author prefers the latter "+" notation but both notations are used interchangeably in photography.