A 5.9 inch shelf sounds tiny until you look at what you actually reach for every day — most of it is narrow anyway. The point of a narrow gap shelf isn't to hold a warehouse; it's to hold the small, daily things that otherwise scatter across the counter or the top of the washer. Here's the honest split between what belongs there and what doesn't.
Things that fit perfectly
Spice jars, cooking-oil bottles, a roll of paper towels, dish soap, cleaning sprays, dryer sheets, a lint brush, coffee supplies. Anything roughly the width of a jar or a bottle sits fine on a 5.9 inch carbon steel shelf and stays reachable because the shelf is right in the seam you walk past anyway. The win is frequency: these are the things you grab on autopilot, so putting them in the gap removes them from the busy surfaces.
Cleaning sprays and dish soap are usually the overflow that ends up under the sink anyway — if that cabinet is where the clutter really lives, sorting under sink storage by what you own is the place to start.
Things that don't belong
Bulky appliances, wide mixing bowls, a full paper-towel multi-pack standing on end, anything tall that would tip when you pull the shelf. A gap shelf is shallow by design — it earns its place by being thin enough to slide into the seam, not by carrying a pantry. Keep the heavy and the tall low and centered so it doesn't shift.
How to decide for your own gap
Stand in front of your fridge or washer and list the five things you touch most while cooking or doing laundry. If most of them are jar- or bottle-shaped, the slim gap shelf is the right call. If they're bulky, you want a rolling cart in open floor instead (more on that below). Measure the seam first — if it's around 6 inches, this shelf is built for it.
Check your seam against the 5.9 in slim gap shelf, or browse narrow storage.