Window coverings are nice to have for privacy, but do they really matter that much when you’re furnishing a room?

Jake Arnold, an interior designer in Los Angeles, offers an unequivocal answer: Yes!

Drapery, he said, is “one of the most important parts of an interior’s layers.”

A great pair of curtains, he added, is “the difference between something that’s OK and something that can make a space incredibly dramatic.”

But choosing drapes is more complicated than you may think. “There are just so many things to consider,” Mr. Arnold said. “There are different styles of windows, the architectural detailing, the windowsill, the hardware.”

So how to get started?

Mr. Arnold recently invited us into a living room he designed in the Hancock Park neighborhood of Los Angeles, featured in his new book from Rizzoli, “Redefining Comfort,” to show us what to do.

Assess the Space

Begin by taking a close look at your room to figure out what kind of window covering you need.

In a living room or dining room, curtains are often the best choice, Mr. Arnold said, as long as there’s enough wall space to pull them to the side, away from the glass, when they’re open.

“For practicality, in kitchens and bathrooms, and on either side of a bed,” he said, “I’ll typically do a Roman shade” — which is raised and lowered vertically — because countertops and night stands can get in the way.

If you decide to go with traditional curtains, think about style.

“The architectural style of a space is really important,” Mr. Arnold said, and should inform how your curtains are made.

“For more contemporary spaces,” he said, “I’m inclined to do a ripple fold” — or a curtain without pleats.

A more formal room may call for drapery with lavish pleats, but for the relaxed, traditional look Mr. Arnold usually prefers, he likes to keep it simple: “I’ll do a single pinch pleat just an inch or two from the top” — in other words, a stitched fold wherever he plans to add a drapery hook for hanging.

That’s what he chose for the Hancock Park living room.

Pick the Fabric

“Fabric selection is key,” Mr. Arnold said.

“There are two approaches,” he continued. “One is where you’re picking something fairly neutral that blends with the paint, which can be really sophisticated and timeless.”

On the other hand, he said, “If the room doesn’t feel layered or there’s nothing particularly inspiring about it, adding a new pair of drapes with a particular color or print could dramatically change the space.”

When you’re reviewing swatches, study the color, pattern and texture, but also think about the weight of the fabric.

“Drapes are almost like a gown,” Mr. Arnold said. “It’s about how the material sits: Is it very structured or does it have a really nice organic flow?”

He sometimes chooses wool or silk, but he especially likes a linen-and-viscose blend: “It’s got that natural nubbiness of a linen, but then the viscose gives it a bit more of a refined sheen.”

For this living room, he chose off-white linen.

Finally, consider the light-blocking qualities of your favorite fabrics. Mr. Arnold almost always lines his drapery with a second fabric, which can be sheer enough to simply filter the light or dense enough to provide full blackout. Sometimes, he adds a pair of sheers behind the main curtains for better light control.

Choose the Hardware

The hardware you hang your drapery on can be concealed or exposed above the window.

For a clean, custom look, Mr. Arnold sometimes likes to hide the curtain rod or track by tucking it behind a valance or crown molding.

Other times, he looks for simple exposed hardware that will blend in with the surrounding décor. In that case, he chooses curtain rods and rings that echo the style and finish of any architectural hardware in the room, like doorknobs and hinges.

“I tend to go away from decorative finials that are very traditional,” he added, noting that he usually prefers simpler options, like curtain rods with flat end caps.

Measure

When you’re ordering drapery, it’s essential to make sure you have the exact measurements.

First, determine how high you’ll mount the hardware.

“I like it four to eight inches from the ceiling,” Mr. Arnold said, to make the room feel taller. If there are beams or other things that prevent that — as in the Hancock Park living room — mount it as high as possible.

Then think about how the drapery will hit the floor.

If it just grazes the floor, it will look straight and tailored. “My rule of thumb, 99 percent of the time, is to do an inch-and-a-half puddle,” Mr. Arnold said, “because it gives it a more relaxed feel.”

It’s possible to add more fabric for a palatial-feeling puddle, but longer curtains will pick up more dirt and can be more difficult to operate.

Finally, consider fullness. Curtains should be double or triple the width of the window, Mr. Arnold said. That way they look generous even when they’re closed.

Install and Enjoy

If you’ve chosen carefully, installation day should be a joy.

The most important thing? Make sure your hardware is secure.

Ideally, curtain rods and tracks should be fastened with screws to studs or blocking in the wall. “Drapery can be very heavy, so it can be dangerous if it’s not hung properly,” Mr. Arnold said.

Getting beautiful curtains can require more time — and money — than you may expect. But the result, Mr. Arnold pointed out, is always worth it: “It’s a missed opportunity if you don’t get it right.”

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