Distill the essence of summer perfection - the freedom to relax and do nothing - into physical form, and you have the hammock.

There's not too much to do in one, except nap or read. Or maybe let your mind wander.

When you're flat on your back and swinging in space, even drinking a glass of lemonade is a challenge. Laptop compatible? No way.

Daniel Mack, author of The Hammock: A Celebration of a Summer Classic (Stewart, Tabori & Chang, $17.95), explains why, in his words, "you get that dumb look on your face when you get in one."

"So much in our world is quantifiable. [The hammock] brings us back to another world, place and time," Mack says in an interview. "That beautiful arch, swing, ellipse . . . it's organic, in a culture that's in love with the mechanical and technical."

Say hammock, and most people picture a white stretch of knotted rope swaying invitingly between leafy trees. The rope hammock is quintessentially American. But it is the relatively young cousin of the centuries-old sleeping sling made first from bark, later from string woven into cloth.

Hammocks are thought to have evolved in the Caribbean from fishing nets. Sleeping on hard ground in a sultry climate invited bugs and dampness. Hanging above ground was drier and cooler, and took better advantage of available breezes. That hammocks were, and still are, light, portable, and easy to set up and take down added to their appeal.

Basically, hammocks come in two materials, cloth or rope, although you can find examples made from eucalyptus wood, hemp, even recycled soda bottles.

Weaving methods and patterns vary by region, but most of the colorful cloth hammocks today come from Mexico or elsewhere in Latin America, and are made completely of cotton. Some are woven string, some are fabric.

Rope hammocks are also traditionally made from cotton. Soft, yielding, and conforming to the body, cotton has one big downside: It rots when exposed to moisture.

Cotton hammocks are still readily available, and purists won't recline in any other kind. But synthetic material - strong, durable and, most of all, rot-resistant - also is used.

The most common types of hammocks are:

Mayan, which is the most popular woven hammock, typically from Mexico's Yucatan Peninsula. Mayan hammocks are woven from cotton string into a thin but extremely comfortable fabric. They provide excellent air circulation and conform to the body. But they can snag or tear.

Nicaraguan, a tighter weave than the Mayan. This makes them harder to tear, while still providing good air flow. Many come with macrame-type fringe.

Brazilian, the thickest, tightest weave of the three, and good for cooler climates. These are the least likely to snag or tear, and come in great color selection and fringe options. (Fringe is believed to keep bugs away.)

Rope, the iconic American hammock, available in cotton or synthetics. Cotton is softer, and it breathes, but synthetics are easier to care for, last longer, dry faster, and don't mildew. Synthetic yarn also comes in fade-resistant colors such as green, blue, red and orange.

"Our best-selling hammock is a rope hammock made from forest green DuraCord [a fabric made of synthetic industrial yarn]. People like it because they don't have to bring it in . . . it can stay outside," says Debbie Regan, manager of Hill Co.'s store in Chestnut Hill.

There are many brands of knotted hammock; the most recognizable names are Pawley's Island and Hatteras, both made in North Carolina.

Nowadays, you can buy a hammock to match your outdoor chair cushions. Made from such fabrics as Sunbrella, these hammocks don't fade or mildew, and come in a range of colors and patterns. Some are quilted for extra comfort.