The hinge is a judgment call. The drawer slide is arithmetic.
Get the hinge family wrong and you'll know it when the door hangs funny. Get the slide spec wrong and you'll know it when the drawer you just built doesn't fit the opening it was built for. The difference is that hinge problems announce themselves before you're committed. Slide problems announce themselves after you've already cut every piece.
Most drawer installation failures don't happen at installation. They happen two steps earlier, when someone built the drawer box to fit the opening instead of to fit the slide.
Here's how to not do that.
Two families, two different geometries
Undermount slides mount to the underside of the drawer box. The slide body attaches to the case floor; a clip bracket attaches to the drawer box bottom. When the drawer is open, you see the box. The slide is hidden. The gap requirement is 16mm total width — the drawer box is 16mm narrower than the opening. Some manufacturers specify 17mm; some European undermounts run 12–14mm. Read the spec for the slide you're actually using.
Undermount is the current standard for furniture-quality work and residential cabinetry. Soft-close is almost always integrated — it's part of the mechanism, not an add-on. The clip receiver on the box bottom constrains how you build the box slightly, but it's not complicated. The installation margin is smaller than side-mount, which means the dimensions have to be right before you start.
Side-mount slides mount to the sides of the drawer box. The body attaches to the case side; the rail attaches to the box side. The gap is bilateral — typically 12.5mm per side, 25mm total. The box is 25mm narrower than the opening. Some slides specify 12mm per side. Read the spec.
Side-mount is the right choice for shop cabinets, utility drawers, file drawers, and tool storage. It's simpler to install, more tolerant of dimensional variation, available in load ratings that no undermount can match at a reasonable price, and not a worse product. It's hardware for a different context. Using undermount slides in a shop cabinet isn't craftsmanship. It's money that doesn't improve the function.
The choice between them is an application decision, not a quality decision.
Extension
3/4 extension means the drawer opens 75% of its depth. The back of the box stays inside the case at full extension. Standard for utility and shop work. Higher load ratings at lower prices because the mechanism is simpler. If you don't need to reach the back quarter of the drawer, there is no reason to pay for full extension.
Full extension means the drawer opens 100% of its depth. The back of the box clears the case face. Required for kitchen drawers where you're pulling out pots or digging through utensils. Required for hanging file drawers. In furniture work: specify full extension when access to the back of the drawer is part of how the piece gets used. Otherwise, 3/4 is adequate.
Over-travel extension is a specialty spec for base kitchen cabinets where the user needs to see into the back without bending. You'll know if you need it.
Spec extension based on access requirement. Not on habit, not on category assumption.
Load ratings
Slide ratings are expressed in kilograms per pair.
50kg: Standard residential kitchen, bedroom, and furniture drawers. Clothing, utensils, cookware.
70–100kg: File drawers, pantry pull-outs, heavy utility cabinetry. Most undermount slides in this range include soft-close as part of the mechanism.
150kg+: Heavy shop drawers, mechanic's tool storage. Side-mount steel slides. Undermount at this rating is specialty hardware and costs accordingly — side-mount is almost always the right call here.
The failure mode for an underrated slide is slow. The balls or wheels in the race compress. The slide loses smoothness. The drawer starts to sag at full extension. By the time the client says something, the box has been transferring load to the bottom edge of the case opening and the slide is binding rather than bearing.
Spec to actual use load. A kitchen drawer that holds cast iron is not a 2kg drawer. Think about what goes in it, not what could theoretically go in it if someone put nothing heavy in it.
The gap spec is the critical dimension
This is where the drawer box gets built wrong.
Every slide specifies the required clearance between the opening width and the box width. This is not a tolerance range. It is a fixed dimension. The slide body is designed for one gap. Build to it.
Undermount: box width = opening width minus 16mm (or whatever the spec says — read it).
Side-mount: box width = opening width minus 25mm (or 24mm, or whatever the spec says — read it).
Height gap for undermount: the slide body needs 16–19mm of clear space between the box bottom and the case floor. That zone must be unobstructed.
The most common error: someone builds the drawer box to fit the opening with comfortable clearance, then tries to make the slide work around it. The slide doesn't care about comfortable clearance. The slide has a specification, and if your box isn't built to it, the drawer won't work.
Build the box to the slide spec. Install the slide to the case. The fit follows from the arithmetic.
Soft-close
Soft-close decelerates the drawer in the last 50–70mm before close. The drawer doesn't slam. It catches and pulls closed.
In undermount slides, soft-close is typically integrated. In side-mount, it's a purchase specification — you choose whether it's included.
Spec soft-close for all residential delivery. It's an expectation of furniture-quality work, not an upgrade. The exception is shop and utility cabinetry where cost is a constraint and a slam close is acceptable. In a shop cabinet, nobody cares. In a client's kitchen, they notice.
Some undermount slides include "self-close" as a separate feature — an internal spring that pulls the drawer to fully closed when it's within 50mm. That's different from soft-close (deceleration only). The full residential specification is soft-close plus self-close. Read the product description.
Installation sequence
The sequence is fixed. Don't reverse it.
1. Establish the reference height for the bottom slide. The lowest drawer sets the reference. Measure from the case bottom to the center of the bottom drawer opening. Everything above derives from there. Set the lower references first.
2. Mount the case members. Side-mount: the slide body mounts to the case side, flush with the front face (or set back for face-frame installations — check the spec). Undermount: the slide mounts to the case floor, positioned per the manufacturer's template. Most undermount slides include a jig card. Use it.
3. Mount the drawer members. Side-mount: the rail mounts to the box side at center height per spec. Undermount: the clip bracket mounts to the box bottom at the specified distance from the drawer front.
4. Test fit before final fastening. Insert the drawer. Check that it opens and closes smoothly at full extension, has no lateral play beyond spec, and that the face is flush with adjacent faces on both sides. Adjust before drilling all the pilot holes.
5. Final fastening and adjustment. Drive all the screws. Set drawer face height with the adjustment screw on undermount slides before lockdown. Center the lateral gap on side-mount before lockdown.
The reason the order matters: case member position is set first. Drawer member position is calibrated to the case member. Final fastening locks both. Reversing this — mounting the drawer members to the box first, then trying to calibrate to the case — compounds error at every step. It doesn't work.
Four questions before you spec:
What's the application — residential furniture or utility/shop? That determines the default family. What's the actual use load — not theoretical minimum, but what goes in it? What access does the user need — full drawer depth or not? And: what is the opening width minus the slide gap spec?
That last number is the drawer box width. Build to it before you cut anything.