Often referred to as the "heel to toe" test, this test begins with the officer placing the test subject into the position of instruction. If there is a painted line or a seam in a side walk to use as reference, the officer may allow the test subject to use this line for reference. If no line is present, the officer will tell the test subject to "imagine" a line on the surface upon which they stand. The officer will tell the test subject to then place the left foot directly in front of the right foot (touching heel to toe) and to keep their hands to their side. The subject is told to remain in this position until given further instructions.
When told to begin the subject is instructed to take nine steps, touching heel to toe, down the line, keep their hands to their sides, look at their feet, and to count out loud. The subject is then told when he reaches the count of nine, to keep that leading foot on the line and take a series of small "stutter" steps in a circle to end up facing the direction they had just walked. The subject is then told to take an additional nine steps, heel to toe.
As stated above, the test subject may never have heard these instructions before in their life but are required to process, remember and perform no less than 7 assignments with no preparation or practice.
Stepping out of line (even if imaginary), failing to touch heel to toe, failing to perform the turn as instructed, failing to count out loud, failing to look at the feet, or using one's arms for balance are all items the officer may use to judge a person is impaired.