You can apply one or multiple strokes to a path using the Appearance panel. For the complete description of the panel's interface, refer the Appearance Panel section.
To open or hide the Appearance panel, use the Window > Appearance command from the main menu.
1 - Strokes are listed in the Appearance panel among other properties.
2 - Add stroke.
3 - Delete stroke.
4 - Stroke width.
5 - Open stroke properties.
6 - Stroke color. Clicking this button opens the Color panel.
Add and Delete a Stroke
Before adding or deleting a stroke, you should select a path in your design.
To add a stroke, click on the ring icon in the bottom bar of the Appearance panel.
To delete a stroke, select it in the Appearance panel. Then click on the trash bin icon.
It is possible to hide a stroke without actually removing it. To do this, use the respective check box in the Appearance panel.
Edit Stroke Width
The Path Width tool lets you customize the Stroke width.
The path that you are going to edit must have a Stroke applied. Its original width doesn't matter. If the most part of the path should finally have certain width, let's say 20 px, you can start from the Stroke width set to 20 px.
To modify the width, select a path, and then activate the Path Width tool. Place the cursor to a point where the width should be changed. A handle will appear on the path near the cursor. It is temporary for now.
Drag the handle off the path to set a new width at the selected point. This point will have three handles. If you drag the handle in the middle, the point, that defines the width, will move along the path. If you drag any of the handles on the sides, you will adjust the stroke width.
While holding down the Alt key, you can move a handle on only one side. This lets you create non-symmetrical strokes.
If you deactivate the tool, the handles will disappear until you activate the tool again.
The Path Width tool lets you manipulate only the stroke width handles. For other operations like selecting, moving or resizing objects, you should activate an appropriate tool.
Properties of a Stroke
The Appearance panel gives you direct access to the width and color of strokes. The Color panel is described in the Color Panel section. To access all available properties, click on the gear icon.
The Width parameter is the same as the width you can set in the Appearance panel.
The Cap options define whether the stroke should end where the path ends, or it should go a half-width farther. In the second case, you can choose if the stroke should be rounded. The option applies to a solid line as well as to individual elements of dashed or dotted lines.
The Corner options let you select how the stroke looks at corners. You can make it sharp, rounded, or clipped.
The Limit parameter lets you set up the mitre limit. It defines the appearance of thick strokes in sharp corners. This parameter has no effect with rounded or clipped corners, as well as when Align is set to the right option.
Notice that by setting certain limit, we can either clip spikes, or let them be as long as they can at the given angle and thickness.
Let's take a closer look at one of the spikes. The path was selected to be able to see its location in relation to the stroke.
With higher limits, spikes at corners can go far beyond the area outlined by the path (compare images above). This is one of reasons why you might want to set some limit.
Clipping takes into account the stroke width and the angle. The wider the stroke or the smaller the angle, the sooner the corner will be clipped at the same level of Limit. In the zoomed in image, the angle of the outer corner is smaller than the inner one. This is why the outer corner was clipped, but the inner one wasn't.
The Align options let you align a stroke to the center of the path, or outside, or inside of the shape. Notice that the alignment options can work only with closed paths.
The following set of three controls lets you create dashed and dotted lines.
In the middle, there is a drop-down menu with different patterns with different combinations of dots and dashes. You can start the creation of that kind of lines by choosing an appropriate pattern.
Any dashed line can be described as a sequence of dashes and gaps of a particular length. So, in order to create your own pattern, you can fill in the dash and gap fields at the top. For example, to create a pattern like "long dash - gap - dot - gap", you can type in 10 - 5 - 1 - 5 starting from the leftmost dash field.
The slider below the menu with patterns moves the pattern along the path. With this slider, you can place dashes at corners of your shape, or align the pattern to other features of the path.
The two pairs of identical controls let you add arrowheads and other elements to path ends. Each set of the controls relates to one of the path ends.
To add an arrowhead to a path, choose a pattern from the drop-down menu. In order to adjust the arrowhead's size, change the numeric value below the drop-down menu.
You can change the profile of a Stroke.
By default, strokes have the Uniform profile which produces lines with constant width. There are other options. For example, the stroke width can gradually change from max to min value along the path.
You can add your own profile to the list if you wish to reuse it. To do this, you should create a new profile using the Path Width tool. Keep an object with the customized profile selected. Bring up the list of profiles, and click the plus button. To delete a custom profile from the list, select this profile, and click on the trash bin icon.