(thing) by bitter_engineer Sun Jul 16 2000 at 5:46:34

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

  H H H H H H H
  | | | | | | |
H-C-C-C-C-C-C-C-H
  | | | | | | |
  H H H H H H H
Other forms of C7H16 are not considered heptane, but are identified by naming the longest string of carbon atoms, then referencing the branches off of them, as in 2-methyl-hexane, below: 
  H H H H H H
  | | | | | |
H-C-C-C-C-C-C-H
  | | | | | |
  H | H H H H
  H-C-H
    |
    H

I like it!