There are several ways to draw a straight line in Concepts.
1. Line Smoothing. The simplest way to draw straight lines is to set your tool’s Line Smoothing to 100%. This turns every stroke you draw into a straight line between your starting point and ending point, no matter how shaky your hand is. To adjust the amount of smoothness applied, tap the smoothing button (the squiggly line) on the inner ring of your tool wheel.
Often, our designers will make a single straight line, and simply duplicate the line to make fast, parallel lines without drawing. You can move, rotate and resize them quickly without needing to draw a million different lines. To duplicate a line, select it and tap the duplicate button on the popup... you’ll see how fast it is.
2. Line Guide. The Line guide gives you straight lines with more precision. The Line guide is Concepts’ ruler - it shows you measurements when Measure in on, and you can customize the length of the line as well.
To find the Line guide, tap the Precision menu on your canvas, then tap the Guide tab and select the Line tool from the popup menu. Turn on the guide by tapping the circle beside the guide tab (so it’s filled in). Position your line where you want it, using one finger on the crosshairs or using two fingers to rotate/resize, or select just one of the circular handles to move the line around a set point. Draw anywhere beside or on the guide, as when guides are active, they’re the only area on the canvas that can be drawn on. To draw elsewhere on canvas again, deactivate the guide.
Also, you can double-tap the cross-hairs at the center of the Line guide to contain the line between the handles (very useful for drawing to exact measurements). To make the line a specific length, tap+hold on the value next to the center of the line and enter the desired length.
3. Line Object. You can import a line object from the Object Library and drag it onto the canvas. Try using the Nudge tool to lengthen or shorten it.
Find an illustrated tutorial on drawing straight lines here.