Highway research board classification system is also known as public road administration (PRA) system.

A triangle, having equal length of sides (i.e. Equilateral triangle) is used in textural classification chart.

Since properties of clay soils is less dependent on particle size distrubution(textural classification) and more dependent on size it is best suited for clay soil.

The purpose of soil classification is to arrange various types of soils in to groups according to their engineering or agricultural properties.

Based on particle size, soil can be classified as

For general engineering purposes, soil can be classified in terms of particle size, textural and unified soil.

Since the textural classification is based on the percentages of sand, silt, and clay size making up the soil. Such a classification is more suitable for describing coarse-grained soil.

To use the textural classification chart, for a given percentage of three constituents forming a soil, lines are drawn parallel to three sides of the equilateral triangle.

HRB classification system, also known as public road administration (PRA) is based on both the particle-size composition and plasticity characteristics.

Particle size classification used only as a designation of particle sizes, and do not signify the naturally occurring types which are mixtures of a particle of different sizes.