I'm not very good in English , so is hard to explain but how put the triangle in the left that deploy or collapse a group of cells ?

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    $\begingroup$ Use SetOptions[EvaluationNotebook[], ShowGroupOpener -> True] to show the opener icon next to the first cell in a cell group. $\endgroup$ – kglr Aug 15 '14 at 20:49
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    $\begingroup$ Also have a look at Opener and 'OpenerView' $\endgroup$ – Jens Aug 15 '14 at 20:57
  • $\begingroup$ Opener ! Thank I look in the documentation but I had no idea how to call it $\endgroup$ – Kafkarudo Aug 15 '14 at 21:03
  • $\begingroup$ I believe it's cucarakafka $\endgroup$ – Dr. belisarius Aug 15 '14 at 21:46
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    $\begingroup$ That's my cousin XD $\endgroup$ – Kafkarudo Aug 15 '14 at 21:48

Several ways to make the opener icon for cell groups visible:

You can set the ShowGroupOpener to True for the current notebook using

SetOptions[EvaluationNotebook[], ShowGroupOpener -> True]

If you want the icon to show in all notebooks in the current front-end session use

SetOptions[$FrontEndSession, ShowGroupOpener -> True]

To get the opener icon showing in all later sessions, use

SetOptions[$FrontEnd, ShowGroupOpener -> True] 

Alternatively, you can use the Option Inspector under the Format menu and change the value of ShowGroupOpener under Cell Options >> Display Options to True

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    $\begingroup$ I suggest you add that you can also do this at the Style Sheet level. +1 $\endgroup$ – Mr.Wizard Aug 16 '14 at 5:18
  • $\begingroup$ @Mr.Wizard, I am drawing a blank on how to do this using StyleSheets. If you don't mind could you please suggest a way or post a seperate answer? $\endgroup$ – kglr Aug 17 '14 at 2:11
  • $\begingroup$ I mean that you can set ShowGroupOpener for a StyleData cell in a style sheet to add an opener to all cells of that style. You could of course also create a new style, inheriting from an existing one if needed, that has an opener, then adding an opener is as easy as setting a style (which can be assigned a keyboard shortcut). $\endgroup$ – Mr.Wizard Aug 17 '14 at 5:38
  • $\begingroup$ @Mr.Wizard and kglr, do you know what is the best way to show the opener only for headings, but not for input/output cell pairs? Is there anything simpler than specifying it in the stylesheet individually for each heading style? $\endgroup$ – Szabolcs Sep 19 '15 at 14:43
  • $\begingroup$ @Szabolcs How about setting SetOptions[EvaluationNotebook[], ShowGroupOpener -> True] and then putting an explicit ShowGroupOpener -> False on the Input style in the stylesheet? Or do you mean something else? $\endgroup$ – Mr.Wizard Sep 19 '15 at 18:42

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