Telescopic drawer slides typically have three sections within each slide. This, for example, allows for a 20 inches extension even when the drawer slides are 20 inches in length when closed. If this was a two section drawer slides it would not be possible to extend 100% of its closed length, the maximum you could get would maybe be 15 inches but the weight capacity would be very low as a result. Next, take into consideration the type of mount and measure out the slide length requirement accordingly. If you are going for side or side-under mount slides, you have to measure the distance from the inside face of the back of the cabinet to the front edge of the cabinet. Then, subtract approximately 1 inch from that for the required slide length. In the case of center slides, just measure the length of the drawer, as the slide length has to be equal to that. However, be aware that there is a problem with this mounting style that we will go into in the next section. It is very important to get the measurements correct. If your slides are shorter than the drawer length, they may not provide enough opening length or adequate support once fully opened. They might even cause the drawer to give out and fall off at some point.