Dear student,
There are  occasions when it is important to be able to show the precise 3-D  arrangement in parts of some molecules.  To do this, the bonds are shown  using conventional symbols:

For example, you might want to show the 3-D arrangement of the groups around the carbon which has the -OH group in butan-2-ol.
Butan-2-ol has the structural formula:

Using conventional bond notation, you could draw it as, for example:

The only  difference between these is a slight rotation of the bond between the  centre two carbon atoms.  This is shown in the two models below.  Look  carefully at them - particularly at what has happened to the lone  hydrogen atom.  In the left-hand model, it is tucked behind the carbon  atom.  In the right-hand model, it is in the same plane.  The change is  very slight.

 












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