I'm making a letter M in Illustrator by using the pen tool and drawing half of the letter and then mirroring it with a transform effect. I noticed that when I zoom in there is a hairline space due to the fact that the edge is not 100% vertical. I've tried moving the anchor point around but I can't get it completely vertical even when holding shift.
You can select two or more anchors and click the buttons on the Align Panel or the align buttons on the Control Panel across the top of the screen (if it's set to align to selection).
Or select the anchor points and choose Object > Path > Average and select the "Vertical" option.