GD&T Perpendicularity Definition, Symbol, Measurement & Perpendicularity vs Squareness


Perpendicularity is an important control in GD&T and CNC design, it can ensure the machined part fit and function as intended. In this article, we’ll focus on perpendicularity definition, measurement, symbol, and the difference between perpendicularity and squareness.

What is Perpendicularity in GD&T?

The perpendicularity in GD&T is a tolerance that specifies the degree to which a surface or feature must be perpendicular to a specified datum plane or axis. It indicates how the orientation of a right-angled part feature can vary if allowed. The tolerance zone for perpendicularity is established by two parallel planes or a cylindrical surface that are perpendicular to the datum reference frame. The distance between these planes or the diameter of the cylinder defines the allowable deviation of the measured feature from perfect perpendicularity. Perpendicularity in GD&T may mean two different things, the first form is surface perpendicularity, which controls perpendicularity between two 90° surfaces, or features. And the second form – axis perpendicularity controls how perpendicular a specific axis needs to be to a datum. 


Perpendicularity is typically represented by a square symbol with a vertical and a horizontal line inside, and the tolerance frame must have both a numerical value for tolerance and a datum. It is commonly used to control the orientation of features such as holes, pins, and mating surfaces, ensuring that they are perpendicular to the mating surface or to the axis of rotation.

The perpendicularity symbol is ⊥ and it is used for both surface and axis perpendicularity.

How to Measure Perpendicularity?

Perpendicularity can be measured with a height gauge, but the gauge is locked to a 90° datum to complete the measurement. If it is a flat surface, the whole surface needs to be measured. If there is a need to measure if the part or feature is axially perpendicular, the maximum material condition is often called out on axis perpendicularity, it can be designed for either a hole or pin feature. Below let’s take a look at some common methods or equipment used to measure the perpendicularity. 

– Coordinate Measuring Machine (CMM): A CMM is a computer-controlled device that uses a probe to measure the surface or feature of an object. A CMM can be used to measure perpendicularity by probing the surface or feature at various points and comparing the data to the nominal value.

– Optical Comparator: An optical comparator is a device that projects a magnified image of a part onto a screen. A user can place a gauge block or reference part onto the screen to check the perpendicularity of the part.

– Surface Plate and Height Gauge: A surface plate is a flat, level plate made of granite or other hard materials. A height gauge is a measuring tool used to measure the height of a surface. A surface plate and height gauge can be used together to measure perpendicularity by placing the part on the surface plate and measuring the distance between the surface plate and the surface being measured at multiple points.

– Square and Dial Indicator: A square is a tool that has a straight edge at a right angle to a flat base. A dial indicator is a precision measurement tool used to measure small distances. The square can be placed against the surface being measured, and the dial indicator can be used to measure the distance between the square and the surface at various points.

Difference Between Perpendicularity and Squareness

Perpendicularity refers to the degree to which a surface, line, or axis is perpendicular to a specified datum plane or axis. It is used to control the orientation of features such as holes, shafts, and mating surfaces.


Squareness refers to the degree to which two surfaces or features are perpendicular to each other. It is represented by a squareness symbol (two perpendicular lines) in GD&T and is used to control the orientation of features such as edges, flanges, and faces.


When it comes to their differences, perpendicularity controls the orientation of a single surface or feature with respect to a reference plane or axis, while squareness controls the orientation of two surfaces or features with respect to each other. Additionally, perpendicularity is usually specified relative to a datum plane or axis, while squareness can be specified between two features without the need for a datum.