Dior Homme, AW11, Paris.
A hook and bar fastening is a popular choice for the closure on tailored trousers, skirts and other garments. It is an alternative to a button closure, particularly where a clean, minimal look is required. It is also a fairly secure fastening that can generally hold more strain than a hook and eye fastening (which is normally just used as a light weight closure where two edges meet, rather than overlap).
The image above from the Dior Homme Autumn-Winter 2011 collection shows the position on mens trousers where this closure is most commonly used, as a sturdy way of fastening the waistband of mens or womens tailored trousers. It could also be used in a similar way on skirt waistbands.
The diagram below shows how the hook and bar are normally positioned for a waistband that has an extended tab such as on the Dior Homme trousers above. The diagram also shows the position of the small internal button, sometimes called a jigger button, that helps to take some of the strain off the hook and bar closure. This internal button must be placed accurately so that the hook and bar can sit closed correctly. If the jigger button is not taking the strain of the waistband, then the hook and bar closure will look as though it is pulling open. 
The image below is from a garment by Tze Goh as seen previously in the post Padded Minimalism by Tze Goh». This shows how the hook and bar can also be used as a minimal closure on another area of the garment as it has been used here to fasten the centre front of the cape as well as the adjustable tabs inside the garment.
Tze Goh, AW10.
There are two different types of hook and bar closure that can be used. The first does not require any sewing but has teeth that slot through the fabric and wrap through the holes in the backing plate to secure it in place. The images below show some of the variations of this type that are available. They are often referred to as 4 part hook and bar, or are sometimes named relating to how many teeth pierce the fabric, such as “2 jaw” or “3 jaw” hook and bar.
4 Part Hook and Bar examples from Made-in-China» and AsiaButton».
The images below show the other type of hook and bar that can be sewn on with small, neat buttonhole stitches around the loops at either end. It is best to test different types on small scraps of your fabric to determine if it will suit the needs of your garment. Bear in mind that you will also have a more secure fastening if the hooks and bars are secured to a strong internal layer, rather than just only being attached to a layer of fabric. So this could mean using strong fusing or a layer of interfacing or canvas as an under layer and attaching your hooks and bars through the fabric to the structure underneath.
Sew on Hook and Bar examples from MacCulloch & Wallis»
Dior Homme images from Vogue.co.uk»
Tze Goh images from TzeGoh.com» and LN-CC»
Technical images from The Cutting Class.
