At the surface of a gas, the pressure force acts perpendicular (at right angle) to the surface.A closely related quantity is the stress tensor σ, which relates the vector force 
  
    
      
        
          F
        
      
    
    {\displaystyle \mathbf {F} }
   to the 
vector area 
  
    
      
        
          A
        
      
    
    {\displaystyle \mathbf {A} }
   via the linear relation 
  
    
      
        
          F
        
        =
        σ
        
          A
        
      
    
    {\displaystyle \mathbf {F} =\sigma \mathbf {A} }
  .
Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. Unlike stress, pressure is defined as a scalar quantity. The negative gradient of pressure is called the force density.Another example is a knife. If we try to cut with the flat edge, force is distributed over a larger surface area resulting in less pressure, and it will not cut. Whereas using the sharp edge, which has less surface area, results in greater pressure, and so the knife cuts smoothly. This is one example of a practical application of pressureFor gases, pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure; such measurements are called gauge pressure. An example of this is the air pressure in an automobile tire, which might be said to be "220 kPa (32 psi)", but is actually 220 kPa (32 psi) above atmospheric pressure. Since atmospheric pressure at sea level is about 100 kPa (14.7 psi), the absolute pressure in the tire is therefore about 320 kPa (46 psi). In technical work, this is written "a gauge pressure of 220 kPa (32 psi)". Where space is limited, such as on pressure gauges, name plates, graph labels, and table headings, the use of a modifier in parentheses, such as "kPa (gauge)" or "kPa (absolute)", is permitted.