The curve that is obtained from winding the hypotenuse of a right triangle one of whose legs is held on a generatrix of the cylinder is called helix to the curve of the cylinder.

The propeller can also be defined as the trajectory of a point that rotates with a uniform movement around an axis moving simultaneously in a direction parallel to the axis with an even uniform movement.

In relation to the previous definitions we can also give two definitions of what is called the helix passage.

According to the first definition, the propeller passage is called the distance between the two closest points that are on the same generatrix of the cylinder.

Based on the second definition of the propeller, we can define the step as the distance traveled parallel to the axis by the generator point while it is turning completely around the axis.

Regardless of the characteristics of the profile and dimensions of a thread, the fillet or protruding part of the thread can be wound in two different directions. If, when looking at the threaded part, the threads or threads of the thread are displaced to the right at the bottom, the thread is called a thread to the right, if, on the contrary, the threads or fillets are displaced to the left For its lower part, the thread is called left hand thread or simply left hand thread.

Normal threads are on the right, so that whenever threads are treated without specifying that these are on the left, it will be understood that it refers to threads on the right.

There are threads formed by more than one thread, two, three or more parallel, wound simultaneously. These threads are called multiple entry threads and the most common are those of two steaks and those of three steaks, called two entrances and three entrances, respectively.

In the threads of a single thread or fillet the pitch is normally equal to the length of the profile generating the threaded surface measured in the direction of the thread axis.

In the threads of several entries, the pitch is greater than the length of the fillet generating profile, the multiple of this length corresponding to the number of entries. Thus, in a thread with two entries, the step p will be twice the step p´ that would correspond to a thread of the same profile with a single entry and in a thread with three entries the length of the step p would be 3 times the smoothness p´ that It would correspond to the same fillet profile if the thread was a single entry.

It can easily be understood from what we have just indicated that the threads of several entries find application in those cases where threads with a thread of relatively small dimensions are desired with respect to the length of the passage, for which reason such threads are sometimes referred to as Fast pitch threads since each turn of the propeller corresponds to a relatively large advance.

For the execution of the threads of several entries, the way in which the different fillets that constitute them are arranged around the axis of the thread is of great interest.

The fillets are arranged on these threads evenly distributed around the shaft. This means that if the thread is of two entrances, the fillets are also arranged longitudinally with respect to the axis when they are displaced 180 ° in a circular position. In the threads of three entrances the longitudinal position of the three fillets is the same when in circular arrangement it is displaced by 120º.