Why 20 minutes fits some jobs better than 15 or 25
The 15 minute timer suits short tasks like a tea steep or a quick tidy. The 25 minute timer matches a full Pomodoro focus block. Twenty minutes sits between them. It is long enough for a real task and short enough that you can see the end. Pick this length when you want a solid block of work without the longer commitment of a Pomodoro round.
Common uses for this exact length include a nap, a reading sprint, a presentation slot, and one section of an exam. A 20 minute nap is a common practice because it ends before you sink into deep sleep. A reading sprint of 20 minutes covers several pages without losing focus. Many conference talks and class presentations are capped at 20 minutes, so speakers rehearse against this clock. Exam sections often run 20 minutes each, which gives you a steady pace across the paper.
The countdown reads from your computer clock, not by counting ticks. That means it stays correct when the tab is in the background or the device sleeps. The tab title shows the time left, so you can watch it from another tab. A tone plays when time is up. You can pick beeps, a chime, or silent with a flashing tab title. A running timer survives a page reload on the same page.