The pile, or nap, of real hand-knotted oriental rugs is often combed or oriented in one direction, a feature of their labor intensive, one-knot-at-a-time/one-row-at-a-time vertical construction on looms. The orientation of the nap forms when rug makers comb and compact completed rows of knots toward the bottom of the loom.
When you are evaluating a rug you can detect this orientation by running your hand over the rug's surface. It will feel sleek or smooth one way and rougher (offering resistance to your hand) the opposite way. You can also see it (as the pictures below demonstrate) by the fact that the orientation causes the rug to reflect light to a greater or lesser degree depending on where you stand relative to the pile. The difference can be profound from one end of the rug to the other; and handmade rugs are known for having a "light side" and "dark side".
A Persian Kashmar rug seen from the light side above, and from the dark side below.
A Persian Yazd rug seen from the light side above, and from the dark side below.
The nap-effect (the change you see from one end to the other) can be minimized depending on how the rug is oriented in your room relative to incoming light. Normally, the effect is reduced by pointing the light side of the rug toward the incoming light. To minimize the difference in the nap-effect from end-to-end: when you are standing on the light side of the rug (the pile feels smooth when you push your hand away from you) the window(s) in your room should be on the opposite side of the rug from you; or if you are standing on the dark side of the rug (the pile feels smooth when you pull your hand across your rug toward yourself) the room's windows should be at your back. (To a lesser degree, rugs also have light and dark edges. If you are standing at the light end of the rug the light edge will be on your left side and the dark edge on your right.)
The nap-effect can vary depending upon rug construction and colors used in the rug's background, border and design motifs (thicker pile and darker colors can show more dramatic differences from light to dark side compared with thinner, lighter colored rugs.) Rugs with textured pile effects (either from shearing or from the use of hand-spun or over- twisted yarns) usually show less nap-effect than those made with conventionally spun, open-tuft yarns. The room in which the rug is placed is extremely important too as a modifier of nap-effect. Rooms with access from only one direction are going to be different from those that are open on more than one side. Rooms with many windows or windows on multiple walls will invite experimentation with a particular rug's nap-effect more than those with windows on a single wall.
The nap-effect will alter how a rug looks in your room compared with how it looks in a rug showroom, or in online photos. If a rug doesn't look right when you first place it in your room, try turning it around to see if that improves your impression. Also, make sure to walk around your rug and view it from all directions to be certain that it looks the way you want in your room. And be aware that the nap-effect will make your rug look different throughout the day, and throughout the year, as the intensity and quality of light in your room changes.
When we deliver rugs, we always turn the rugs around in the room to allow our customers to see how the nap-effect changes and allow them to determine if they have a preference for how the rug should be placed so that it looks best to them in their room based on its architectural features, lighting and layout. If you are taking rugs from stores yourself on a trial basis, you should experiment with this in your home throughout the day under different lighting conditions. (The fact that big rugs make this a strenuous exercise is a big reason that online rug-shopping isn't a big hit with consumers.)
Sometimes, the nap-effect can be useful because your rug can look like two different rugs. You can arrange your room in a way that takes advantage of the softness and subtleties of the light side or the rich contrasts of the dark side for optimal presentation with your room's decor.