Pins
Slender metal shafts used to temporarily hold fabric layers together before stitching, in a range of lengths and thicknesses for different fabric weights.
Not all pins are the same pin, even though they can look interchangeable in the box. Fine, short pins (often called silk pins) are for lightweight and delicate fabrics — anything heavier and they bend before they'll go through cleanly. Long, thick quilting or pleating pins are for bulkier fabric and multiple layers, where a fine pin would just get lost or bend under the resistance. Glass-head pins can be pressed over directly without melting the head, which plastic-head pins absolutely cannot handle — I learned that one the hard way, with a little fused plastic blob on the sole of my iron.
The habit that matters more than pin quality: pinning perpendicular to the seamline, not parallel to it, with the head hanging off the fabric edge, so you can pull each pin out just before the needle reaches it rather than sewing over it. Sewing directly over a pin is a good way to snap a machine needle, or worse, throw a bent piece of one flying.
I keep a magnetic pin dish rather than a pincushion at my regular sewing spot — dropped pins on the floor are a genuine hazard, and a magnet sweeps them up far faster than patting around on the carpet.