C20H42, recognized as a string of 20 simply-linked carbon atoms, like so:
  H H H H H H H H 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-C-C-C-C-C-C-C-C-H
  | | | | | | | | | | | | | | | | | | | |
  H H H H H H H H H H H H H H H H H H H H
It is also called icosane.

Other forms of C20H42 are not considered icosane, but instead are identified by naming the longest string of carbon atoms, then referencing the branches off of them, as in 2-methyl nonadecane, below:

  H H H H H H H 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-C-C-C-C-C-C-C-H
  | | | | | | | | | | | | | | | | | | |
  H | H H H H H H H H H H H H H H H H H
  H-C-H
    |
    H

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