Larger sieve openings (1 in. to 1/4 in.) have been designated by a sieve "mesh" size that corresponds to the size of the opening in inches. Smaller sieve "mesh" sizes of 3 1/2 to 400 are designated by the number of openings per linear inch in the sieve.
The following convention is used to characterize particle size by mesh designation:
a "+" before the sieve mesh indicates the particles are retained by the sieve;
a "-" before the sieve mesh indicates the particles pass through the sieve;
typically 90% or more of the particles will lie within the indicated range.
For example, if the particle size of a material is described as -4 +40 mesh, then 90% or more of the material will pass through a 4-mesh sieve (particles smaller than 4.76 mm) and be retained by a 40-mesh sieve (particles larger than 0.420 mm). If a material is described as -40 mesh, then 90% or more of the material will pass through a 40-mesh sieve (particles smaller than 0.420 mm).
This information is also provided on page T848 of the Aldrich 2003-2004 Catalog/Handbook of Fine Chemicals.