You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a double hinge structure utilized for connecting a first unit and a second unit of a folder-style electronic device, and more specifically, to a double hinge structure utilized for making the first unit rotate to a position directly on top of the second unit or to a position directly underneath the second unit.  
           [0003]    2. Description of the Prior Art  
           [0004]    General folder-style electronic devices, such as note-books, portable DVD players, or cellphones, use hinges to connect the main body of the devices to a corresponding screen. Hence, the devices can be opened or closed in a folding manner. Please refer to FIG. 1. FIG. 1 is a three-dimensional diagram of a hinge used in a folder-style electronic device according to the prior art. Please refer to FIG. 2 and FIG. 3. FIG. 2 is a three-dimensional diagram of a closed folder-style electronic device  9  that uses a hinge according to the prior art. FIG. 3 is a three-dimensional diagram of a side view of a closed folder-style electronic device  9  that uses a hinge according to the prior art. When the folder-style electronic device  9  is folded, the screen  91  is positioned directly on top of the input area  92 . When folded, the folder-style electronic device  9  maximizes space savings and is convenient for a user to carry. Please refer to FIG. 4. FIG. 4 is a three-dimensional diagram of an open folder-style electronic device  9  that uses a hinge according to the prior art. When the folder-style electronic device  9  is opened, a user can input instructions or perform operations using the input area  92 . The screen  91  displays results for the user to confirm. However, folder-style electronic products according to the prior art can only rotate 180 degrees, i.e. the angle between the screen  91  and the input area  92  can at most be 180 degrees (please refer to FIG. 5 and FIG. 6). As such, this is a limitation when a user is using a folder-style electronic product.  
         SUMMARY OF INVENTION  
         [0005]    It is therefore an objective of the claimed invention to provide a double hinge structure utilized in folder-style electronic devices (eg: notebooks, portable DVD players, cellphones) to solve the above-mentioned problem. The double hinge structure connects a first unit and a second unit of the folder-style electronic device and is utilized for making the first unit rotate to a first position directly on top of the second unit or to a second position directly underneath the second unit. The double hinge structure comprises a plurality of bolts; at least a plate installed and rotatable on the plurality of bolts; and a plurality of fasteners installed and rotatable on the plurality of bolts.  
           [0006]    These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0007]    [0007]FIG. 1 is a three-dimensional diagram of a hinge used in a folder-style electronic device according to the prior art.  
         [0008]    [0008]FIG. 2 is a three-dimensional diagram of a closed folder-style electronic device that uses a hinge according to the prior art.  
         [0009]    [0009]FIG. 3 is a three-dimensional diagram of a side view of a closed folder-style electronic device that uses a hinge according to the prior art.  
         [0010]    [0010]FIG. 4 is a three-dimensional diagram of an open folder-style electronic device that uses a hinge according to the prior art.  
         [0011]    [0011]FIG. 5 is a three-dimensional diagram of an open folder-style electronic device that uses a hinge according to the prior art when the angle between the input area and the screen is 180 degrees.  
         [0012]    [0012]FIG. 6 is a three-dimensional diagram of a side view of an open folder-style electronic device that uses a hinge according to the prior art when the angle between the input area and the screen is 180 degrees.  
         [0013]    [0013]FIG. 7 is a three-dimensional diagram of a double hinge structure according to the present invention.  
         [0014]    [0014]FIG. 8 is a diagram of a plane view and a side view of the fastener locations and the housing of the double hinge structure.  
         [0015]    [0015]FIG. 9 is a diagram of a side view of a screen directly on top of an input area.  
         [0016]    [0016]FIG. 10 is a diagram of a side view of an open folder-style electronic device after the screen is rotated 180 degrees from the position directly on top of the input area where the screen is above the input area.  
         [0017]    [0017]FIG. 11 is another diagram of a side view of an open folder-style electronic device after the screen is rotated 180 degrees from the position directly on top of the input area where the screen is below the input area.  
         [0018]    [0018]FIG. 12 is a diagram of a side view of an open folder-style electronic device when the screen is rotated to a position directly underneath the input area and the angle between the input area and the screen is substantially 360 degrees.  
         [0019]    [0019]FIG. 13 is a three-dimensional diagram of a folder-style electronic device that uses the double hinge structure of FIG. 7.  
         [0020]    [0020]FIG. 14 is a three-dimensional diagram of another type of double hinge structure according to the present invention.  
         [0021]    [0021]FIG. 15 is a three-dimensional diagram of a folder-style electronic device that uses the double hinge structure of FIG. 14.  
         [0022]    [0022]FIG. 16 is a three-dimensional diagram of another type of double hinge structure according to the present invention.  
         [0023]    [0023]FIG. 17 is a three-dimensional diagram of a folder-style electronic device that uses the double hinge structure of FIG. 16.  
         [0024]    [0024]FIG. 18 is a three-dimensional diagram of a folder-style electronic device that uses a double hinge structure viewed from a first view.  
         [0025]    [0025]FIG. 19 is a three-dimensional diagram of a folder-style electronic device that uses a double hinge structure viewed from a second view.  
         [0026]    [0026]FIG. 20 is a three-dimensional diagram of a folder-style electronic device that uses a double hinge structure viewed from a third view.  
         [0027]    [0027]FIG. 21 is a three-dimensional diagram of a folder-style electronic device that uses a double hinge structure viewed from a fourth view. 
     
    
     DETAILED DESCRIPTION  
       [0028]    The following embodiments according to the present invention relate to a portable DVD player. However, the present invention can be used in all kinds of folder-style electronic devices, such as notebooks, folder-style digital cameras or cellphones.  
         [0029]    Please refer to FIG. 4. FIG. 4 is a three-dimensional diagram of an open folder-style electronic device  9  that uses a hinge according to the prior art. The folder-style electronic device  9  comprises a screen  91  and an input area  92 . A user can input instructions or perform operations through the input area  92 . The screen  91  displays results for the users to confirm. Please refer to FIG. 7. FIG. 7 is a three-dimensional diagram of a double hinge structure  8  according to the present invention. The double hinge structure  8  comprises bolts  81  and  82 ; fasteners  83 ,  84 ,  85  and  86 ; and a plate  87 . The fasteners  83 ,  84 ,  85 ,  86  and the plate  87  are installed and rotatable on the bolts  81  and  82 . When using different types of double hinge structures, the number of the fasteners and plates can be varied for different double hinge structures. Please refer to FIG. 8. FIG. 8 is a diagram of a plane view and a side view of the fastener locations  95 ,  97 ,  96 ,  94  and the housing of the double hinge structure  93 . The fasteners  83 ,  84 ,  85 ,  86  are attached to the corresponding fastener locations  95 ,  97 ,  96 ,  94  of the screen  91  and the input area  92  respectively. Hence, the screen  91  is connected to the input area  92 . The plate  87  is attached to the corresponding housing of the double hinge structure  93 .  
         [0030]    Please refer to FIG. 9. FIG. 9 is a diagram of a side view of a screen on top of an input area  92 . When the screen  91  is connected to the input area  92  using the double hinge structure  8  in the above-mentioned description, because of the functioning of the double hinge structure  8 , the screen  91  faces downward (direction C) with respect to the input area  92 . In this position, the folder-style electronic device  9  is folded, and the screen  91  is directly on top of the input area  92 . The fasteners  83 ,  84 ,  85 ,  86  are parallel and toward the left (direction A), and the fasteners  83  and  86  are directly above the fasteners  84  and  85 . Please refer to FIG. 10. FIG. 10 is a diagram of a side view of an open folder-style electronic device  9  after the screen  91  is rotated 180 degrees from the position directly on top of the input area  92  where the screen  91  is above the input area  92 . After the screen  91  is rotated 180 degrees from the position on top of the input area  92 , the screen  91  faces upward (direction D), and is on the right side of the input area  92  (direction B) and above the input area  92 . The fasteners  83 ,  86  are toward the right (direction B), and the fasteners  84 ,  85  are toward the left (direction A). The fasteners  83 ,  86  are above the fasteners  84 ,  85 . Please refer to FIG. 11. FIG. 11 is another diagram of a side view of an open folder-style electronic device  9  after the screen  91  is rotated 180 degrees from the position directly on top of the input area  92  where the screen  91  is below the input area  92 . Now the screen  91  faces upward (direction D), and is on the right side of the input area  92  (direction B) and below the input area  92 . The fasteners  83 ,  86  are toward the right (direction B), and the fasteners  84 ,  85  are toward the left (direction A). The fasteners  83 ,  86  are below the fasteners  84 ,  85 . Please refer to FIG. 12. FIG. 12 is a diagram of a side view of an open folder-style electronic device  9  when the screen  91  is rotated to the position directly underneath the input area  92  and the angle between the input area  92  and the screen  91  is substantially 360 degrees. In this position, the folder-style electronic device  9  is folded, and the screen  91  is facing downward (direction C) and is positioned directly underneath the input area  92 . The fasteners  83 ,  84 ,  85 ,  86  are parallel and toward the left (direction A), and the fasteners  83  and  86  are below the fasteners  84  and  85 . The screen  91  has been rotated 360 degrees. Please refer to FIG. 13. FIG. 13 is a three-dimensional diagram of a folder-style electronic device  9  that uses a double hinge structure of FIG. 7.  
         [0031]    Please refer to FIG. 14, FIG. 15, FIG. 16 and FIG. 17. FIG. 14 is a three-dimensional diagram of another type of double hinge structure according to the present invention. FIG. 15 is a three-dimensional diagram of a folder-style electronic device  9  that uses the double hinge structure of FIG. 14. FIG. 16 is a three-dimensional diagram of another type of double hinge structure according to the present invention. FIG. 17 is a three-dimensional diagram of a folder-style electronic device  9  that uses the double hinge structure of FIG. 16.  
         [0032]    Therefore, if a folder-style electronic device uses the double hinge structure according to the present invention, the screen  91  can be rotated to a position directly on top of the input area  92  or directly underneath the input area  92 . This means the double hinge structure  8  provides a 360-degree rotating function. This is in contrast to all folder-style electronic products based on the prior art, which do not provide a 360-degree rotating function because their screens can at most be rotated 180 degrees. (Which makes the angles between the input areas and the corresponding screens at most 180 degrees—please refer to FIG. 5 and FIG. 6). Please refer to FIG. 18, FIG. 19, FIG. 20 and FIG. 21. FIG. 18 to FIG. 21 are three-dimensional diagrams of a folder-style electronic device  9  that uses a double hinge structure viewed from a first, a second, a third, and a fourth view, respectively. The 360-degree rotating function of the double hinge structure allows a folder-style electronic device  9  to have a more changeable view angle. Any side of the folder-style electronic device  9  can be used as a holder. When the screen  91  is rotated to the position directly underneath the input area  92 , users can still input instructions through the input area  92  and the screen  91  can still display information. In this working position, the folder-style electronic device  9  saves the most space.  
         [0033]    Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

Summary:
The present invention provides a double hinge structure utilized in folder-style electronic devices (eg: notebooks, portable DVD players, cellphones). The double hinge structure connects a first unit and a second unit of the folder-style electronic device and is utilized for making the first unit rotate to a position directly on top of the second unit or to a position directly underneath the second unit. The double hinge structure contains a plurality of bolts; at least a plate installed and rotatable on the plurality of bolts; and a plurality of fasteners installed and rotatable on the plurality of bolts.