Now that we know how to start sketching, we will learn about what defines a sketch. Constructing a sketch will require two elements: the first is the sketch entities, such as lines, arcs, and circles, while the second is the dimensions and relations that define the sketch entities. Remember that SOLIDWORKS is a form of engineering software that aims to support the design and manufacture of products. Thus, defining shapes is very important.

To define sketches, we can use dimensions and relations. Let's start by defining them:

• Dimensions: These represent distances and angles that can be defined with a number. Some examples are as follows:
  • Lengths of lines
  • The diameters and radii of circles and arcs
  • An angle between two lines
• Relations: These represent geometric relations between the different parts of a sketch. Some examples are as follows:
  • A line can have a horizontal relation to the sketch plane
  • Two lines are perpendicular to each other
  • Two circles are concentric to each other
  • A line is a tangent to a curve

To illustrate this, let's look at a visual comparison between two lines. One is Fully defined and one is Under defined:

Of the two preceding lines, the one on the left is fully defined while the one on the right is under-defined. The line on the left is defined as such because of the following:

• One end coincides with the origin of the coordinate system
• The length of the line was defined as 50 millimeters
• The angle between the line and the x-axis is 45 degrees

These dimensions and relations are what make a sketch defined. It may not look like there's much of a difference between them when we look at the two lines, assuming that we drew them on paper. However, when we work with a computer program, we need to let the computer know everything about the line (or any other sketch entity); otherwise, it won't know what we want.

To make it easier to distinguish between fully defined and under defined sketches, SOLIDWORKS color-codes them. Black parts are fully defined, while blue parts are under defined. In addition to color-coding, SOLIDWORKS also indicates the status of our sketch below the canvas. The following screenshot shows an indication of a Fully Defined sketch. Other classifications include Under Defined and Over Defined:

Sketches are defined with measurements and relations, where measurements refer to numbers and relations refer to geometrical references. We will explore geometrical references in the next section.