Plasticity Chart, developed by Arthur Casagrande (1932) is a plot of the Plasticity Index (PI) versus the Liquid Limit (LL) of soils. The chart is used for the classification of fine-grained soils (or fine-grained fraction of coarse-grained soils) based on their plasticity.
The plasticity chart comprises of two important lines, A-line and U-line. A-line is an empirically chosen line which separates the chart between clays and silts; soils that fall above A-line are classified as clays and that falling below as silts. A-line is given by the equation:

PI = 0.73 - (LL - 20)

U-line lies above the A-line and is approximately the upper limit of the relationship of PI to LL, for any currently known soil. The equation of U-line is given as:

PI = 0.9 . (LL - 8)

There is also a vertical line in the plasticity chart which corresponds to a liquid limit of 50% and separates the high plasticity fine grained soils (LL>50%) from low plasticity fine grained soils (LL<50).

The soil fall above A-line in plasticity chart are clayey soils and that fall below A-line are silt and organic soils.

The following annotations are used to characterize soil on the plasticity chart:

CL: Inorganic Clay with low plasticity (Lean Clay)
ML: Inorganic Silt with low plasticity
OL: Organic soil with low plasticity
CH: Inorganic Clay with high plasticity (Fat Clay)
MH: Inorganic Silt with high plasticity (Elastic Silt)
OH: Organic soil with high plasticity

If plasticity index ranges between 4-7%, soil is represented in dual symbol. That is CL-ML.

Plasticity classification based on PI:
• (0) - Non-plastic
• (< 7) - Slightly plastic
• (7 - 17) - Medium plastic
• (> 17) - Highly plastic

Plasticity classification based on LL:
• (<35 %) - Low plasticity
• (35-50 %) - Intermediate plasticity
• (50-70 %) - High plasticity
• (70-90 %) - Very high plasticity
• (>90 %) - Extremely high plasticity

It should be noted that fine-grained soil samples change from solid to liquid as follows with increasing the wetting or water content:

And the plasticity index PI is defined as:

PI = LL - PL

Shrinkage Limit (SL) is the water content where further loss of moisture will not result in more volume reduction.

Plastic Limit (PL) is the determined by rolling out a thread of the fine portion of a soil on a flat, non-porous surface. The procedure is defined in ASTM Standard D 4318. If the soil is at a moisture content where its behavior is plastic, this thread will retain its shape down to a very narrow diameter. The sample can then be remolded and the test repeated. As the moisture content falls due to evaporation, the thread will begin to break apart at larger diameters.

Liquid Limit (LL) is conceptually defined as the water content at which the behavior of a clayey soil changes from the plastic state to the liquid state. However, the transition from plastic to liquid behavior is gradual over a range of water content and the shear strength of the soil is not actually zero at the liquid limit.