The Pencil Tool (Y) at first sight is similar to the Brush Tool, but actually functions along different mechanics. While the brush tool can be used to both draw lines and crudely fill in areas, a pencil can only create lines.
When a stroke is drawn with the brush, Flash is actually tracing the thickness of the brush, placing appropriate anchor points around the outline, then filling the space with colour. However, Flash traces the centre of a pencil stroke, then creating the colour around it rather than inside it.
In the tools panel, you can control the pencil mode, which determines the smoothness of your stroke. Again, like with the brush tool, Flash interferes to make the most efficient line. With Straighten, Smooth and Ink options, you dictate how much Flash interferes or how much it simply follows your hand.
While the Pencil tool is selected, the Properties panel will contain
multiple options that will let you control the width and general
appearance (style, cap and join) of the line.
Style: The general appearance of the line. By default, the line is a solid straight line, but the drop-down menu will let you change the line so it's dashes, dotted... When clicking on the pencil button next to the drop-down menu, further editing options will come up that let you determine the frequency of dashes, the randomness of the ragged line...
Cap: The ends --or caps-- of the lines. The cap can be either rounded or squared.
Join: The way lines meet, which can be mitered, rounded or beveled.