# What is the difference between flat, straight, level and smooth?

Shop for Engineer’s Straight Edges

Engineer’s straight edges are used to check surfaces are straight. This is important for when you have two surfaces in contact with each other such as on an engine block and cylinder head. In order to achieve a suitable fit, both surfaces need to be straight. However, a straight surface is not the same as a level or smooth surface.

## Flat or straight surfaces

When a surface is flat or straight, it means that no one area is sitting higher than another. In other words, when the surface is looked at under the microscope, all the peaks are at the same height and evenly spaced out over the surface. This does not mean that it is smooth or level.
A surface can be flat but not smooth or smooth but not flat. For example, a sanding block has a flat surface for sanding, but would not be described as having a smooth surface as the grit particles make it rough.

Likewise, a snooker ball has a smooth surface but is certainly not flat.

A smooth surface can still have peaks that prevent it from being flat, these will just be less pointed and sharp, and more rounded and smooth.

### Is it possible to ever get a completely flat surface?

The flatness of a surface is measured by the difference between the highest and lowest points over the length of the surface. Any flat surface looked at under high enough magnification will have a degree of mountains and valleys. Even grade 00 straight edges are not completely flat.

### What is the difference between straight and level?

It is possible to have a straight or flat surface that is not level. This is because flatness or straightness is the measure of the surface variation over the entire length of an object, whereas being level is the elimination of any height difference between two ends of an object.