Im trying to draw a horizontal guideline but Sketchup only seems to want to let me draw vertical ones. No matter what orientation I have the simple 3D block oriented in, or what edge/point I start the tape measure from, every time I click a second time I get a vertical aligned guideline. Is there a specific key or operation I have to do to draw a horizontal guideline after drawing 3 vertical ones?
Dragging the Tapemeasure tool off [or back on to] a horizontal axis OR off a horizontal edge will make a matching ‘horizontal’ guideline.
When dragging off an edge do not let the inference lock to its mid-point etc, otherwise the guideline will be perpendicular to the edge, rather than parallel to it…
Yet its not doing that - hence my post 
Dragging off any edge - horizontal or vertical results in a vertical guideline.
You can constrict the movement of the tape measure on any one of the axes if you also use the keyboard’s arrow keys. So, if your horizontal guideline has to be parallel to the green axis, click the tape measure tool on the green axis line and begin moving the tool from left to right, following the red axis. As you move the cursor, tap the right arrow key; that will constrain the movement and you should see a red dotted line following the cursor. The red color indicates that the cursor is moving along the red axis.
Similarly, if the horizontal guideline must be parallel to the red axis, click the cursor on the red axis line, tap the left arrow key, and begin moving he cursor from front to back, following the green axis line. And if you want a vertical guideline–one parallel to the blue axis, use either the up or down arrow key to constrain the movement.
Hope this helps.
I have had it constrained in the red direction ( right arrow key) then needed to place a line in the green and it would not respond ( left arrow). I am not very strong with this either and had to un-select the tool and re-position and use SHIFT to constrain it to the green. WEIRD?? Good Luck this may shed some light…
I fully understand the locking of the axis/direction by using the shift key. What Im saying is that no matter what direction I drag the tape measure in, from whatever horizontal or vertical surface, the guideline always appears in the vertical. Im going to grab a screencast video to show this.
Have you got a stuck key locking the direction ?
Can you drag off the red/green axes ??