We specialize in stair runners, and your question is a very common one. Often staircases whether large or small, new or old have varying widths. Our advice is always to maintain the same width of the runner on a straight staircase for a couple of reasons. First, attempting to change the runner width will more than likely draw more attention to the very issue you're concerned about. There will be a quirky change in width from one tread to the riser above. Second, the runner itself would need to be custom made or fabricated to accommodate the two widths. This can be done, but adds expense. There are runner products available in 2 different widths. The problem with using two widths in a given product is that the scale of the pattern will often change, scaling up the design elements to the larger width. So, using both will again be quirky and draw more attention. The suggestion above, using tape to determine the desired width is a good one. Just keep in mind you may have to adjust that a bit to the existing width of the runner you like. Hope this helps.