The pull up bar is one of the most elegant and straightforward gadgets in a gym. 

It’s a straight bar mounted at a height. 

You can hang on it. Do chin ups, pull ups, muscle ups or leg raises on it. 

You have three ways to grip this bar:
1 Thumbs pointing to each other (palms away from you or prone grip) 
2 Thumbs away from each other (palms towards you or supine) 
3 Mixed grip. One palm towards you, one palm away from you. 

4 You will find neutral grips on most bars. This allows for your palms to face each other. 

While doing pull work, all these grip positions have an impact on a variety of things:

To what degree, different muscles in the arms, back and shoulders get involved in the movement. Biceps kick in more on chin ups (2). The brachioradialis kick in more on neutral grip pull ups (4). These muscles are less involved in a strict pull up that is achieved with a prone grip
(1). 

The ease with which your scapula moves during the movement. This varies from person to person. Your grip changes your intitial shoulder position. This affects how your humerus (upper arm bone) is positioned in your scapula (glenoid fossa) affects the ease with which you can depress, retract and rotate your scapula as move up on a vertical pull.

The efficiency and control with which you can move your scapula will determine how well you recruit all the muscles in your upper back including the lattismus dorsi to get the pulling done. 

Simply put, the grip can change your comfort and control with which you can perform the movement. 

We are toying with gripping an object shaped like an X. My daughter calls it a spider. It puts the head of the humerus in a much more favourable position in the fossa. This allows for clients to move their scapula more freely and with greater control. This makes for a better vertical pulling experience! 

The bar is two rods positioned perpendicular to each other. This is distinct from a neutral grip that puts two bars parallel to each other. 

While it might not sound like a revolution or dramatically different position. Subtle changes like this have a dramatic effect on how the move feels.