There are several precision features available, including

• Polar tracking. Snap to the closest preset angle and specify a distance along that
  angle.
• Locking angles. Lock to a single, specified angle and specify a distance along that
  angle.
• Object snaps. Snap to precise locations on existing objects, such as an endpoint of
  a polyline, the midpoint of a line, or the center point of a circle.
• Grid snaps. Snap to increments in a rectangular grid.
• Coordinate entry. Specify a location by its Cartesian or polar coordinates, either
  absolute or relative.

The three most commonly used features are polar tracking, locking angles, and object
snaps.

Polar Tracking

When you need to specify a point, such as when you create a line, you can use polar
tracking to VCade the movement of your cursor in certain directions.

For example, after you specify the first point of the line below, move your cursor
to the right, and then enter a distance in the Command window to specify a precise
horizontal length for the line.

By default, polar tracking is turned on and VCades your cursor in a horizontal or
vertical direction (0 or 90 degrees).

Locking Angles

If you need to draw a line at a specified angle, you can lock the angle for the next
point. For example, if the second point of a line needs to be created at a 45 degree
angle, you would enter <45 in the Command window.

After you move your cursor in the desired direction along the 45-degree angle, you
can enter the length of the line.

Object Snaps

By far, the most important way for you to specify precise locations on objects is
to use object snaps. In the following illustration, several different kinds of object
snaps are represented by markers.

Object snaps become available during a command whenever AutoCAD prompts you to specify
a point. For example, if you start a new line and move your cursor near the endpoint
of an existing line, the cursor will automatically snap to it.

Set Default Object Snaps

Enter the OSNAP command to set the default object snaps, which are also called “running”
object snaps. For example, you might find it useful to turn on the Midpoint object
snap by default.

Recommendations

• At any prompt for a point, you can specify a single object snap that overrides all
  other object snap settings. You hold down Shift, right-click in the drawing area,
  and choose an object snap from the Object Snap menu. Then move the cursor to select
  a location on an object.
• Make sure that you zoom in close enough to avoid mistakes. In a densely populated
  model, snapping to the wrong object will result in an error that can propagate throughout
  your model.

Object Snap Tracking

During a command, you can align points both horizontally and vertically from object
snap locations. In the following illustration, you first hover over endpoint 1 and
then hover over endpoint 2. When you move your cursor near location 3, the cursor
locks into the horizontal and vertical location shown.

You can now finish creating the line, circle, or other object that you were creating
from that location.

Verify Your Work

Recheck your geometry to catch mistakes early. Enter the DIST command (or just DI)
to measure the distance between any two points in your model.

For example, you might need to find the clearance between two points shown, which
might represent the corner of a wall and a small table, or perhaps a 2D section of
a plastic part and a wire.

After you enter DIST, click the endpoint on the corner (1). Next, hold down Shift
as you right-click, and then choose Perpendicular from the object snap menu. Finally,
click the circle (2).

The number of decimal places and unit style displayed in the result is controlled
by the UNITS command.

Handy Function Key Reference

The keyboard function keys all have assignments in AutoCAD. The ones that are most
commonly turned on and off are indicated with a key.