Tangent Line Tool

As the Tangent Line tool has several keypresses which can add or change its functionality, we suggest installing the free Astute Graphics plugin Astute Buddy, which creates a panel that dynamically updates to inform you of the various keys which can be pressed in the tool’s current context.

Creating a line tangent to one existing path

A line may created that is tangent to an existing path by either starting on the path and drawing the line away from it, or by starting at another location and drawing the line to the path. Either way, after clicking on the first location, the mouse button can be released and a second click made on the second location, or the cursor can be dragged to the second location. The first method allows scrolling, zooming, changing the view mode, etc. in the middle of the operation, but the second method allows additional keypresses that only work with the mouse button down (see below).

When the cursor is snapping tangently to an existing path, the snap point will be highlighted with a small, semi-transparent green circle. A small letter “P” will also be drawn if the cursor is snapping to an anchor point, and an “M” when the cursor is snapping to the midpoint of a straight path segment.

Tangent Line Tool - From an Existing Path

When drawing a tangent line away from a path, the Shift key can be held down to constrain the line’s length to “nice” values, dependent on the current zoom level and units. For example, at 200% zoom, its length is constrained to multiples of 2 pt or 0.5 mm. Additionally, if the cursor is being dragged (mouse button down), Command/Ctrl can be pressed to override snapping to a second path.

Tangent Line Tool - Drawing to an Existing Path

When drawing a tangent line to an existing path, if the path contains multiple spots of tangency, then the one closest to the cursor is chosen.

Creating a line tangent to two existing paths

If the line is started along a path, and the cursor is subsequently brought close to another path, then, if the line can be positioned so it is tangent to both paths, it snaps to the second path (unless being dragged and Command/Ctrl is being pressed) and the line’s start and end positions are both adjusted. If there are multiple places where a suitable line can be created, then the spot which ends closest to the cursor is chosen.

Tangent Line Tool - Line Between Two Paths

Sometimes it is necessary to create a line which starts at a specific spot along a path and is tangent only
to the second path. In this case, pressing the key assigned to “Increase Diameter” in the native Keyboard Shortcuts dialog (by default, for English-speaking locales, the right bracket key “]”) will tell the Tangent Line tool to ignore the start path when finding tangencies; the initial green circle snapping annotation will turn gray to indicate that the line is no longer being forced to be tangent at the start point. Pressing the key again will return to the normal two-tangencies mode.

Tangent Line Tool - Ignore Starting Path

You can switch between the Tangent Line tool and the Perpendicular Line tool (except when dragging) by pressing the key assigned to “Decrease Diameter” in the native Keyboard Shortcuts dialog (by default, for English-speaking locales, the left bracket key “[”).

Doubleclicking the Tangent Line tool in the toolbox (or pressing the Enter key when the tool is selected) will bring up its preferences dialog, which it shares with the Perpendicular Line tool:

Tangent Line Preferences Dialog

1. Annotate Line Length

When enabled, the current length of the tangent line is displayed next to the cursor.

2. Snap Tolerance

The tolerance for the cursor when snapping to existing paths. If Smart Guides are turned on, the Smart Guides tolerance value is used if it is larger than the specified value.

3. Snap When Smart Guides are Off

When enabled, and Smart Guides are turned off, the cursor will still snap to paths using the specified Snap Tolerance.

4. Add Points to Existing Paths

When enabled, anchor points will be added to the existing path(s) at the points where the tangent line begins or ends. A dot appears next to the cursor to indicate that this mode is active. The preference may be inverted on the fly by pressing the Option/Alt key when using the tool.

5. Perpendiculars Bisect Corner Points

When enabled (the default), corner anchor points are considered to have a perpendicular which bisects the angle made by the path at that point (and its tangent is therefore at 90° to the perpendicular); otherwise, the perpendicular is made to the angle of the path entering or leaving the point.

Tangent Line - Bisect Preference

6. Ignore First Path When Starting at Corner

When enabled (the default), and a path-to-path line is begun at a corner anchor point, the first path is ignored for the purposes of making the line tangent to it. To ignore the first path even when not starting on a corner point, use the Increase Diameter keyboard shortcut key (see Tangent Line: Tool Operation).

7. Informational area

Shows a brief description of each preference control when the cursor is being hovered over it.