Open a mechanics set and half the sockets are marked with fractions — 3/8, 7/16, 1/2 — and half with plain numbers — 10, 13, 17. They are not alternatives to each other. They are two measuring systems, and using the wrong one is the fastest way to round off a nut.

What the numbers mean

Metric sockets are stated in millimetres across the flats of the fastener: a 13 mm socket fits a nut measuring 13 mm face to face. SAE sockets are stated in inches as fractions: 1/2 inch is about 12.7 mm.

That 0.3 mm gap between a 13 mm and a 1/2 inch socket is exactly the problem. The SAE socket will slip onto a 13 mm nut and feel almost right, then round its corners the moment you apply force. Almost-fitting is worse than not fitting, because it lets you start.

Which half you will use here

Vehicles and appliances sold in India are overwhelmingly metric. Two-wheelers, cars, furniture fittings, plumbing hardware — metric. The SAE half of an imported set is there because the set was designed for a market where older vehicles and some machinery use inch fastenings.

This is worth saying plainly: if you are buying a large imported mechanics set for Indian vehicle work, you will probably use less than half of the sockets. That is not a fault in the set, but it should shape what you pay for. A metric-focused set with fewer total pieces may serve you better than a bigger dual-system chest.

Drive size is the other number

Sockets also carry a drive size — 1/4, 3/8 or 1/2 inch — describing the square hole the ratchet plugs into. This one is stated in inches everywhere, including on metric sets, and it is not a measurement of the fastener.

• 1/4 inch — small fasteners, interior trim, electronics, tight spaces
• 3/8 inch — the general-purpose size; most household and two-wheeler work
• 1/2 inch — wheel nuts, suspension, anything needing real torque

If a set lists a 3/8 inch drive, every socket in it plugs into that ratchet. Adapters exist to cross between drive sizes, and they are useful, but stepping a 1/2 inch job down to a 1/4 inch ratchet asks more of the small tool than it can safely give.

A habit that saves sockets

Seat the socket fully before pulling. Most rounded nuts are rounded because the socket was sitting proud on the fastener’s chamfer rather than down on the flats. Push it home, then pull steadily — not in jerks.