The commonly used Plasticity Index (PI) test disposes the material retained on the 425 micron sieve, thus is not representative of the whole sample. In Australia, residual soils are very common, with a high granular content in “clayey” soils. Thus a significant portion of the sample is discarded for the PI (only) test. The weighted plasticity index (WPI) accounts for the portion used in the PI test and this is important for classification of residual soils. The background and historical development of the WPI is presented with its relationships, applications and limitations. Changes in testing Standards affects the correlations expected with the original WPI classification.