A [Tk] (or rather [Ttk]) [widget] that stacks a bunch of widgets on top of each other in a window, with some tabs at the top to allow users to select between the widgets. A common idiom in Windows's Properties dialogs.
    :   http://www.tcl.tk/man/tcl8.5/TkCmd/ttk_notebook.htm

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A [notebook] may be used as a '''pages manager'''[http://wiki.tcl.tk/17852] by using a style to turn off the tabs, like this (by [Joe English]):
    style theme settings default {
        style layout Plain.TNotebook.Tab null
    }
    ttk::notebook $nb -style Plain.TNotebook

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[tonytraductor]  I have a question.  Using a ttk::notebook, how do I pass the currently selected tab's info to a process? 

Answer: [[$nb select]] returns the widget name of the currently selected pane.  In addition, the string "current" may be used as a tab identifier.
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[Flame] A [notebook] can change its tab position like this:
    ttk::style configure ENNotebook.TNotebook -tabposition en
    ttk::notebook .nb -style ENNotebook.TNotebook
----
[Flame] Here is some code to add tab list scrolling to a [notebook]. The code is not optimized, supports only ne-oriented tabs, and assumes user does not change tab states.
    namespace eval SNotebook {

      # create scroll buttons  
      foreach anchor {n s e w} arrow {uparrow downarrow leftarrow rightarrow} {
        set uanchor [string toupper $anchor]
        ttk::style layout ${uanchor}Button.TButton [list Button.focus -sticky nswe -children [list ${uanchor}Button.$arrow -sticky nswe]]
      }
      
      variable scrollpos
      variable hsizes
      variable tabheight
      
      proc _updatehsizes {nb} {
        if {![winfo exists $nb.escroll]} {
          ttk::button $nb.escroll -style EButton.TButton -command [string map [list \$nb $nb] {
            if {$::SNotebook::scrollpos($nb)>0} {
              incr ::SNotebook::scrollpos($nb) -1
              ::SNotebook::_scroll_notebook $nb
            }
          }]
        }
        if {![winfo exists $nb.wscroll]} {
          ttk::button $nb.wscroll -style WButton.TButton -command [string map [list \$nb $nb] {
            if {$::SNotebook::scrollpos($nb)+1<[llength [$nb tabs]]} {
              incr ::SNotebook::scrollpos($nb) 
              ::SNotebook::_scroll_notebook $nb
            }
          }]
        }
        variable hsizes
        variable tabheight
        set ntabs [$nb index end]
        set tabs [$nb tabs] 
        ttk::notebook .tmp -style [$nb cget -style]
        set p0 [winfo reqwidth .tmp]
        set hsizes($nb) $p0
        for {set i 0} {$i<$ntabs} {incr i} {
          set tab [lindex $tabs $i]
          if {1} { ;# theme-independent, but could have side effects 
            eval .tmp add [ttk::frame .tmp.f$i] [::SNotebook::$nb tab $tab] -state normal
            update idletasks
            set hsizes($tab) [expr [winfo reqwidth .tmp]-$p0]
            set p0 [winfo reqwidth .tmp]
          } else { ;# there is a theme-dependable constant, which includes padding and tab margins
            eval ttk::label .tmp2 -style TNotebook.Tab [dict remove [::SNotebook::$nb tab $tab] -sticky] 
            set hsizes($tab) [expr [winfo reqwidth .tmp2]+10]
            destroy .tmp2
          }
        }
        destroy .tmp
        ttk::label .tmp -style TNotebook.Tab
        set tabheight [winfo reqheight .tmp]
        destroy .tmp
      }
      
      

      proc _scroll_notebook {nb} { ;# assumes tabposition is nw or ne
        variable scrollpos
        variable hsizes 
        variable tabheight
        if {![info exists scrollpos($nb)]} {
          set scrollpos($nb) 0
        }
        set startindex $scrollpos($nb)
        for {set i 0} {$i<$startindex} {incr i} {
          ::SNotebook::$nb hide $i
        }
        set ntabs [$nb index end]
        set tabs [$nb tabs] 
        
        set availw [winfo width $nb]
        set reqw $hsizes($nb)
        set overflow 0
        for {} {$i<$ntabs} {incr i} {
          set tab [lindex $tabs $i]
          incr reqw $hsizes($tab)
          ::SNotebook::$nb add $tab
          if {$reqw>$availw} {
            incr i
            set overflow 1
            break
          }
        }
        for {set j $i} {$j<$ntabs} {incr j} {
          ::SNotebook::$nb hide [lindex $tabs $j]
        }
        set h $tabheight
        
        set eh [expr 4*$h/5]
        if {$startindex>0} {
          set ew [expr $eh*[winfo reqwidth $nb.escroll]/[winfo reqheight $nb.escroll]]
          place $nb.escroll -x 0 -y 0 -width $ew -height $eh
        } else {
          place forget $nb.escroll
        }
        if {$overflow} {
          set ew [expr $eh*[winfo reqwidth $nb.wscroll]/[winfo reqheight $nb.wscroll]]
          place $nb.wscroll -relx 1.0 -x -$ew -y 0 -width $ew -height $eh
        } else {
          place forget $nb.wscroll
        }
      }
      
      proc snotebook {path args} {
        ttk::notebook $path {*}$args
        _updatehsizes $path
        bindtags $path [linsert [bindtags $path] 1 SNotebook]
        bind SNotebook <Configure> {::SNotebook::_scroll_notebook %W}
        rename ::$path ::SNotebook::$path
        proc ::$path {cmd args} [string map [list \$path $path] {
          switch $cmd {
            index -
            configure -
            cget -
            identify -
            instate -
            select -
            state -
            tabs {
              ::SNotebook::$path $cmd {*}$args
            }
            default {
              bind SNotebook <Configure> {}
              set res [::SNotebook::$path $cmd {*}$args]
              ::SNotebook::_updatehsizes $path
              bind SNotebook <Configure> {::SNotebook::_scroll_notebook %W}
              ::SNotebook::_scroll_notebook $path
              return $res
            }
          }
        }]
        return $path
      }

    }

    ::SNotebook::snotebook .nb
    pack .nb -fill both -expand true

    set i 0
    foreach theme [ttk::themes] {
      frame .f$i
      pack [button .f$i.b -text "Theme: $theme" -command "ttk::setTheme $theme"] -anchor nw
      .nb add .f$i -text "Tab $i"
      incr i
    }
    for {} {$i<20} {incr i} {
      .nb add [text .t$i] -text "Tab $i"
    }
----

!!!!!!
%|[Category Widget]|%
!!!!!!