WHY 3/4 EXTENSION SLIDES?
Quick Answer- To give you the best bang for the buck. Full Extension slides are roughly 3 times the cost of 3/4 Extension...3/4 Extension will do the job in the application we are using them for.
To answer this...first I want you to go to your knife and fork drawer and open it. Notice, it is right under your countertop...next to no overhead clearance. Yet, I am willing to bet dollars to doughnuts, the drawer is only 3/4 extension. If there was ever a place for full extension...this is it. So, why did they use 3/4 extension slides? Certainly a main consideration was the cost (subsequent affordability) of the slides themselves. But a second consideration is the application. What do you store in there? Knives, forks, and spoons...all of which lay flat and can be accessed from the front of the drawer...there is no need for full access. If the drawer was used to store small jars of face cream...totally different story...you need full access.
Now go over to your lower kitchen cabinets and open any door. What do you see? Two shelves...a bottom shelf that is part of the cabinet...and a middle shelf...about a foot higher...which can be fixed or moveable. These are the shelves you want to convert to rollouts....each has about a foot of overhead clearance. So, in this application...you may WANT full extension...but you do not NEED full extension. Once installed, the top of the drawer is about 4" above the shelf...so, you have about 8" of overhead clearance to allow you to reach and extract any item from the back of the drawer.