An open stance is one where the line from toe to toe is perpendicular to the path of the ball. The term closed stance has some variation in usage. In the EASI Tennis System, we define a closed stance as one where the line from toe to toe is parallel to the path of the ball. In Bollettieri's terminology, this is a semi-closed stance. Yandell's usage of these terms appears to refer to the line from shoulder to shoulder rather than from toe to toe. 

In keeping with our objective to make every aspect of our system clear, we have separated the issue of the shoulder alignment at contact from the foot alignment, or stance, at contact. This gives rise to four possible cases: open stance with open shoulder, open stance with closed shoulder, closed stance with closed shoulder, and closed stance with open shoulder. All of these configurations can be found in professional tennis, as well as many gradations in between. For the purpose of organizing our information, we do not break down the discussion further but leave it to you to experiment with gradations.

We now illustrate these four possibilities. In each case, the illustration on the left is the takeback position and the illustration on the right is an interactive image showing the transition from takeback to strike. The intermediate stages are omitted to focus attention only on the issues of stance and configuration.

The first illustration is the open stance with open shoulders at contact. We call this the Open-Open stance. This is a widely used technique. A key point with this stance is understanding how to efficiently get the elbow in front of the body plane. This will be discussed in a later lesson. The rotation stage consists primarily in using the legs and waist to bring the racquet into alignment with the path of the oncoming ball. Another feature of this technique is the use of the rotation stage as a major source of acceleration, thus nearly combining the two stages. Note the use of the legs to assist in the rotation stage.
 

The next illustration shows the open stance with closed shoulders, the Open-Closed stance. This technique is not as common as the previous one, but it does occur in professional play. The takeback position is nearly the same as with the Open-Open stance, but the following stages differ, as can be seen by passing the cursor over the image on the right.
 

In the Open-Closed stance, the shoulder is utilized to produce acceleration by being contracted forward. Thus the legs are less utilized, resulting in a lower total expenditure of energy than is found in the Open-Open stance.

In the Closed-Closed stance illustrated below, the player's shoulder is pointing more toward the oncoming ball. There are wide variations on this stance: the toe-to-toe line can range from parallel to the oncoming ball to up to 45 degrees from its path.
 

Agassi uses this stance often. However, Agassi resolves the problem of getting the elbow in front of the body plane by using a takeback position in which the elbow is already in front of the body plane. There are numerous examples of Closed-Closed stance takeback positions in which the elbow starts behind the body plane as illustrated above, as well as in front of the body plane. A discussion of this is found in Rotation.

Another common stance is the Closed-Open stance illustrated below.
 

In this stance, the shoulders at contact are perpendicular or even greater than 90 degrees rotated relative to the oncoming path of the ball, as can be seen by moving the cursor over the illustration at the right. The legs are utilized to assist both rotation and acceleration.

You should experiment with all of these techniques. Any given pro will use any of these stances depending on the circumstances. It is a fact that you will learn faster by exploring well-defined alternative methods of performing a task than you will by trying to focus on a single method.