In mechanical drawing you may encounter a need to meet several conditions simultaneously
When designing a part you might need to size or position features to satisfy more than one constraint. For example you might need to position:
a bolt hole tangent to some edge and also, when the part is rotated 45 degrees, tangent to another edge.
the center of a hole on the circumference of a circular part and, when the part is moved by 1 inch, on some edge .
a circle tangent to two other circles
a circle tangent to a circle and a straight line
You can accomplish these types of design challenges by repeatedly experimenting with different positions until the result is close to what is desired. But that method is time consuming and inaccurate.
Instead, create temporary guidelines and use the intersection feature. For example:
To position a circle A tangent to circles B and C:
Create a temporary circle concentric with B but use a radius equal to the sum of the radius of circle A and B. The center of circle A will need to coincide with this temporary circle.
Create a temporary circle concentric with C but use a radius equal to the sum of the radius of circle A and C. The center of circle A will need to coincide with this temporary circle also.
Position A at the intersection of the temporary circles.
Delete the temporary circles.
To position a hole centered between two items horizontally and also between two items vertically:
Draw a straight line representing the horizontal center.
Draw a straight line representing the vertical center.
Snap the hole to the intersection of the two straight lines.
It helps to actually try the above steps to understand how it works.
In the video below, we've designed a jig with a flat plate. Watch how perpendicular lines are used to define the constraint.