The basic hand position protecting chin and body. Its height and width determine which punches get through at all.
The standard guard keeps both hands at chin-to-cheek height, elbows tucked to the body, chin slightly down, eyes looking over the gloves. The two most common mistakes are a lead hand carried too low — then every straight lands — and elbows flaring out, leaving the body open. Also important: a guard isn't static. Pure blocking hands your opponent the tempo; good defence combines the guard with head movement and footwork. And a detail many miss — when you punch, the other hand stays up. Guards mostly open during your own attack, not during defence.