Meet the father of two with the power to determine the cut and finer details of your clothes.

John Gallagher is undressing behind a sheet of white cardboard in a conference room in Manhattan’s Flatiron District. After a minute or two, the 54-year-old emerges in a pair of yellow boxer briefs. ‘‘The fly opening needs to be higher up,’’ he says, as the designer of Mack Weldon, a boutique underwear label, takes notes. Then Gallagher points to the pouch. ‘‘See right here? There needs to be more room here to allow for fabric shrinkage.’’

Gallagher is a fit model, which is essentially a living mannequin, someone who tries on clothes so that designers can evaluate their shapes and cuts before runway models introduce them to the world. But he is also more than a fit model; he is the fit model, an unlikely expert consultant recruited by companies — which have included Brooks Brothers, Ralph Lauren and Thom Browne — to articulate how much further up a waistband should hang on the hips, or how much deeper a pocket needs to be. ‘‘The designer approaches fashion from a style point of view,’’ he says. ‘‘My job is to make sure you can move your arms in a suit.’’ Weighing in on these small yet critical details has made Gallagher so popular that some 70 labels now hire him to test out their wares.