Skinny as a rail, or simply narrow in build, like a rectangle this body type is straight up and down. More pointedly, the waist and the shoulders are the same width apart. This distinction makes this shape deviate slightly from an average, or athletic build, where the hips and waist are narrower than the shoulders and chest. While this doesn't necessarily mean that you're skinny or undefined, it's more of a shape that's less likely to show muscle definition, or simply appears narrow.
The name of the game for this figure is to add definition where this is none. Adding this definition via clothing can be done in a few ways. Layering is really the simplest way to begin. With a smart layered knit, you're adding to the upper half, while emphasizing your neutral waistline. Add a jacket to create depth and visual strength in the shoulders. These things will make you look muscular, even if you're stick skinny.
When it comes to bottoms, you should opt for a skinnier pant. This way, you're creating a inverted triangular line that goes from your chest to your bottom—much like the inverted triangle type. Fortunately, considering this figure is generally lean looking, you're able to be flexible with what you wear, and still look skinny.
However, be careful when buying things off the rack, as brands that provide more generous cuts—whether they're T-shirts are—will leave you looking swallowed up by your clothing if you're not careful. You can (and should) look for “slim fit” items, especially when it comes to buttondowns and suit jackets. While other guys may pass on structured blazers, look for something that fills out your shoulders and chest.
And before you think that your trim physique means you don't need to see the tailor, think again. Most clothing off the rack (particularly suits) will fit in one section, but swallow up your arms or waist. To avoid looking boxy and unbalanced, considering taking your jackets to a tailor to have the sleeves narrowed. The result will be a strong upper body, with the sculpted torso and slender leg that makes this silhouette distinctive.