Ask any plain clothes law enforcement officer or daily concealed carry civilian what type of holster they use most often and 9 out of 10 will tell you that they are using an In-The-Waistband Holster (IWB Holster) on a daily basis.
The major advantage of the IWB holster is the fact that the lower half of the gun is inherently concealed inside the trousers. This small but significant detail might seem trivial, but ask anyone who has carried a handgun for more than 6 months and they'll tell you that it's much easier to choose clothing and outerwear that will conceal a pistol if only the grip section has to be concealed.
Strong side holsters are most popular but Appendix carry holsters positioned inside the pants as well as crossdraw style IWB holsters are also available for those looking for alternative carry options.
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A few disadvantages are brought up by those who prefer a duty holster or shoulder holster include the comfort factor, the need to purchase pants in sizes slightly larger than normal, and minor difficulties getting into and out of a vehicle. The majority of these factors, however, are shared with other holster styles that will satisfactorily transport a full-sized handgun and all of those who choose to equip themselves with an IWB holster tend to feel that the benefits outweigh the associated hindrances.
It's because of this overwhelming popularity that most holster manufacturers offer more than one IWB Holster design. Most hand-made leather holster craftsmen like those at Milt Sparks or Lou Alessi have a number of IWB holster designs. Not as many Kydex holsters are made for carry in-the-waistband because the material does not lend well to the application but there are a few if you look long enough.