Families – Lines

This post is a continuation from last weeks posts on families in particular some items the tutorials don’t cover.


If reference planes are the skeleton and labelled dimensions are the muscles of the family, reference lines are the joints in the skeleton. They define which way the parts of the skeleton can bend (rotate) but they can also do more.

Start and end points

As I mentioned in last weeks post, reference lines have a start and end point, unlike reference planes. This provides additional data to Revit when you are trying to use angular dimension parameters in a family. When constraining a reference line to a reference plane as shown in the image (Fig. 1) below, the vertex point of the rotation is not the intersection of the reference line and the reference plane. It is either the start (1) or end (2) point of the reference line.

Figure 1

In this example Revit assumed that the start point (1) of the reference line was meant to keep it’s relative position to the two reference planes which means that the end point (2) will move when the “r” parameter is changed. This can be changed however by locking the end point (2) to the reference planes with locked dimensions. Then the start point (1) will change location.

While we’re on the topic, the length of a reference line can also be adjusted using parametric dimension or by locking the end points to reference planes. This is how line based families work. I’ll cover parametric dimensions more in a later post.

Associated work planes

Another major difference between reference planes and lines is the number of work places associated with them. A reference plane only defines one work plane which can be named in the properties palette. A reference plane has four work planes associated with it. See figure 2 below.

Figure 2. A selected reference line in a 3D view.

A reference line has 2 work planes that run along the length of the line and intersect one another along that line. There is also a work plane at the start and end points of the line that are perpendicular to the line. To specify which work plane you want to model or draw on start the “Set work plane” command (Select the Home->Work Plane->Set icon) and select “Pick a Plane” from the popup dialog. Then with your mouse over the reference line press the tab key to cycle through the work planes on the line. You need to be over the start or end point to select that work plane. Drawing on these work planes allows those parts to rotate with the reference line, since the work planes rotate with the line.


These posts will get complicated real fast and I’ll do the best to keep them simple. The best way to learn this stuff is to open Revit and try out what I’m talking about. In the next post, I’m going to go over parametric dimensions including automatic dimensions which might help explain some of those mysterious outcomes you have when flexing a model for the first time.

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