text: Measurement Tools 3 eLearning | Dimensioning & Tolerancing (GD&T) Training | Amatrol
  The Measurement Tools 3 eLearning course provides a comprehensive education on General Dimensioning & Tolerancing (GD&T), which is a system used to describe tolerances for more complex geometric features. Sample topics include: placing a datum feature symbol on a drawing, describing the reasons why GD&T is used, and using position tolerances for the location of multiple features. After giving learners an overview of GD&T, the curriculum focuses on three main areas: Location Tolerances; Orientation Tolerances; and Form Tolerances.
  What is Conventional Tolerancing? What are Bilateral and Unilateral Tolerances?
  A tolerance is the total amount by which a specified dimension is allowed to vary. One way to specify a tolerance is to show the plus and minus tolerance value next to nominal dimension. This is called conventional tolerancing. There are two ways conventional tolerances are specified on a drawing: bilateral and unilateral tolerances.
  A bilateral tolerance means that the actual dimension can be either larger or smaller than the dimension shown on the drawing. If the amount of variation in either direction is the same, the bilateral tolerance is indicated. In some cases, the tolerance of a dimension must be made smaller in one direction than another because of the design needs of the part or the assembly into which the part is being assembled. To deal with this, two tolerance numbers are used.
  In certain cases, the dimension of the part can only be allowed to vary in one direction. The amount that the dimension can vary in this one direction is called a unilateral tolerance.