Since the 1913 edition of Webster's dictionary, the nomenclature of hydrocarbons has become more specific in regards to structure. Dodecane is now specifically a string of twelve linked carbon atoms, like so:

  H H H H H H H H H H H H
  | | | | | | | | | | | |
H-C-C-C-C-C-C-C-C-C-C-C-C-H
  | | | | | | | | | | | |
  H H H H H H H H H H H H
Other forms of C12H26 are not considered dodecane, but are identified by naming the longest string of carbon atoms, then referencing the branches off of them, as in 2-methyl-undecane, below:
  H H H H H H H H H H H
  | | | | | | | | | | |
H-C-C-C-C-C-C-C-C-C-C-C-H
  | | | | | | | | | | |
  H | H H H H H H H H H
  H-C-H
    |
    H