There are two common ways to place a buttonhole on a garment: horizontally and vertically.
Once you know the purposes of these two different orientations, you can decide which one would work best for your project.
Horizontal buttonholes are ideal for areas that will receive stress. For example, if you are making a blouse with a front or back button placket, you might want horizontal buttonholes so that the buttons can have some left-to-right movement as you move. This will help eliminate the gapping that can occur on blouse button plackets, especially those located at shoulder level in the back or at the bust in the front. Other areas that benefit from horizontal buttonholes are the top neck button and cuffs of a man’s dress shirt.
Vertical buttonholes are ideal for areas of low stress, or in cases of vertical stress. Examples include a man’s dress shirt front placket, a pocket, and the closure on a tote bag or purse.
Of course you can always choose how and where to place your buttonholes. The most important consideration of all is simple: how you want it to be. Enjoy what you sew and the choices you make. It is a wonderful thing to be able to create a project just the way you want!