The square head genno has rounded corners for strength, narrows slightly at the waist and is noted for its excellent directional stability.
The 115g (4oz) square gennou hammer is a lovely light hammer with a long handle, ideal for delicate fine adjustments.
The heavier hammers are ideal for assembly work, knocking in pegs and nails and also make a very acceptable choice for driving Japanese chisels and setting Japanese planes.
For chisel work the 375g (13.2oz) size is recommended.
The polished faces are selectively hardened by quenching the head under a stream of flowing water, this gives durable working surfaces but leaves the middle of the head softer, which deadens the blow and kills vibration. The polished hachimaki bands around the faces are not a laminated layer but a traditional decorative touch that tidies up the appearance after polishing the faces.
Handle: Japanese White Oak
| weight (g) | head dimensions (mm) | overall length (mm) |
| 115 | 60 x 17 x 15 | 300 |
| 225 | 75 x 20 x 23 | 327 |
| 300 | 85 x 25 x 22 | 325 |
| 375 | 90 x 30 x 25 | 330 |
| 450 | 96 x 31 x 25 | 360 |
| 570 | 103 x 33 x 30 | 360 |
| 670 | 103 x 36 x 30 | 360 |
