Tommy John and Stanntt are joining forces.

The underwear and custom shirt brands will host a joint pop-up at the Roger Smith Hotel in New York starting March 29 and running through March 31.

This is the first pop-up partnership for Tommy John, a men’s underwear and ath-leisure line that’s sold at department stores including Bloomingdale’s, Nordstrom and Dillard’s.

“A partnership between Stantt and Tommy John is a natural fit,” said Tom Patterson, who founded Tommy John in 2008. “Most guys that wear dress shirts also wear an undershirt. Just as many women don’t leave home without a bra, most guys don’t leave the house in a dress shirt without also wearing an undershirt. With Tommy John, Stantt shirts look, feel and fit the best way possible.”

Stanntt, which is sold at Saks Fifth Avenue, uses three measurements and its patented DataFit Technology to match a customer to one of 75 shirt sizes. At the pop-up, customers will be fitted for a shirt and able to sample a Tommy John undershirt. For every three shirts purchased, customers will receive a free Tommy John Second Skin undershirt, and for every two shirts purchased customers will receive a free pair of Tommy John socks. The custom shirts will be shipped to customers in a little over a week. Shoppers can also order additional Tommy John items — undergarments and ath-leisure — from tablets in-store and meet the founders of both brands on March 31 from 6 to 9 p.m.