The Shinwa digital angle meter is used for measuring angles relative to horizontal, (or vertical) and relative to each other in degrees or % (handy if you prefer straightforward ratios to trigonometry).
I have seen similar devices variously described as a level box, angle finder, digital angle gauge and even inclinometer (which it isn't because an inclinometer also has some form of sight attached) either way, I'm sticking with digital angle meter.
As with most digital devices, there are cheap and cheerful mechanisms and good ones, (the layout of the buttons gives you a clue) so the sweet spot is a good one at a reasonable price.
Housed in a beautifully cast aluminium body with twin magnets on both sides and the base, the Shinwa measures angles in degrees or % incline to 1 decimal place.
If used upside down (i.e. stuck to the ceiling) the readout inverts so you can still see the measurement.
When first switched on the word LEVEL will appear on the display showing that you are measuring relative to horizontal.
Pressing the zero button sets the current angle as zero and the device will then measure variance from that angle (turn off and on again to return to measuring from horizontal).
Pressing hold locks the display at the current reading.
Pressing the percent degree button switches between readout modes.
Supplied complete with screwdriver for removing the back plate to change the battery (one of those 9v rectangular ones with both terminals on one end).