The term “fitting” a sight basically means you are adjusting the size of the sight to make it properly fit the dovetail of the firearm you are trying to install the sights on. Most aftermarket sight manufacturers will make their sights slightly oversized so they will fit most dovetails even if the dovetail has loose tolerances. If you have a tight dove tail and an oversized sight you will have to cut the sight down to fit the dovetail. When you fit a sight you have to pay close attention to the fit--measure twice and cut once. Cut slowly and test fit often. If you don't have the experience and don't pay careful attention, you can easily cut too much or in the wrong area . Once you cut off more than you need to, the sight will be too loose in the dovetail and can easily fall out. This is an example of a customer who took off so much material, the sights are now unusable. This is an unfortunate $100.00 mistake that can easily be avoided with care and know how. On top of ruining the sights, the slide was also damaged during the install. The customer had used a Dremel Tool and cut the slide several times. 

This is what was mailed to us, with a note asking for help. Above from left to right: Picture one is a side view of the dovetail with the customer fitted sight. So much material was removed you can actually see through the dovetail and sight (2 red arrows). Middle: upon visual inspection you can see the marks of a Dremel Tool. The red and blue arrows point out were the rotary tool slipped when the person tried to cut the stock sights out and nicked the finish. Right: the sight was so loose i was able to remove it with finger pressure alone. When I removed the sight, it was revealed the Dremel Tool had actually cut the slide as well.

These two pictures illustrate how much material was removed and the angles involved. In the left picture above the blue lines show what the angle the sight should have been cut at. The red lines show the angles where the customer actually cut the sight. The right picture is showing the angles of the wrongly “fitted" sight compared to a factory fresh sight.

These pictures show just how rough the sight was cut. Remember if you are fitting the sight take the extra step and do a final polish of the sight where you cut. A smooth surface and some oil will make your aftermarket sights glide into place without scratching your slide's factory finish. Remember, the goal of installing aftermarket sights is to make it look like a factory installation.