It’s coming today to a phone in your pocket or a laptop on your desk — online sports betting for residents 21 or older is legal in Massachusetts for the first time ever. In-person sports betting, also for those 21 or older, was approved in January and available at the three Massachusetts casinos: Encore Boston Harbor, Plainridge Park in Plainville and MGM Springfield.

Online betting for 18-year-olds is allowed in Montana, New Hampshire, Rhode Island and Washington state.

Now, the state-approved sportsbooks will be allowed to go live Friday and accept wagers online. The state has already approved 10 sportsbook licenses to date and allows a total of 15 licenses overall. Massachusetts will also be taking a 20% tax bite out of sportsbook revenues; 15% of revenues earned at in-person sites.