Let it be known: When it comes to city pools, “tankinis” are in, while tube socks are very much out.

The District’s parks and recreation department on Monday issued detailed guidance on the proper attire for a visit to the city’s two dozen public swimming pools. Long story short: “proper swim attire” is necessary to swim or even tread upon the pool deck.

That means no “Brazil/French-cut, thong style and/or revealing swim wear, cut-off jeans, jeans, skirts, shorts, sport bras, leotards, leggings, dri-fit wear, compression shorts or compression shirts.” And don’t even think about wearing underwear with your swimsuit.

Wear “street clothes” or other prohibited items, department spokesman John Stokes said, and pool staff will ask you to leave the premises.

Stokes says the detailed guidance was prompted not by one particular incident [see update below], but a need to remind the swimming public of what does and does not fly in the context of public aquatics. “We were getting so many questions from our lifeguards, and they were seeing so many residents just wear whatever out on the pool decks, we felt it was really necessary to give them more detail about pool attire,” he said.

Improper attire, the department suggests, can “transport airborne and ultimately water borne contaminants into the pool,” while absorbent fibers like cotton can, when worn in the pool, “break down in the water and cause fibers to clog filters ” while also absorbing chemicals needed to keep the water clean. Dyes in non-pool-approved shirts can bleed into pool waters, affecting “water chemistry and balance.”

The official guidance on poolside fashion comes about two months after controversy erupted over a health department rule banning all food and drink, including water bottles, from pool decks. Faced with an uproar from thirsty swimmers, health officials clarified that water bottles are allowed on pool decks, though not at pool’s edge.

Meanwhile, three indoor city pools remain out of service. The aquatic centers at Wilson in Ward 3 and Deanwood in Ward 7 have been out of service since last week due to air conditioning failures, Stokes said, while the pool at Ferebee-Hope Recreation Center in Ward 8 remains closed due to a leak in the pool’s lining.

Stokes did not have an estimate for reopening those facilities but noted four other indoor pools and all of the city’s 19 outdoor pools remain open.

Update, 4:35 p.m.: The policy update comes less than a month after Allison Stewart, a Turkey Thicket pool user, blogged about her frustrations after being asked to leave — and later thrown out and banned from the pool by police — over what pool staff believed was a non-conforming shirt. A petition started by Stewart asking the parks department to clarify its policy and better train staff garnered 182 signatures.