If you were going to buy one item for an instant update of just about anything in your closet, a challis skirt would do it. Put it with a blazer, a pullover sweater or a shirt and the effect is a softer look, which is the current pitch of fashion after several seasons of mannish tailoring.

Challis, by dictionary defintion, can be sheer worsted, cotton, rayon or other fibers or blends made in a firm plain weave.It can be solid, but this year has shown up frequently in paisley or foulard prints. Because of its lightweight quality, it is ideal for Washington's mild fall climate.

Yves Saint laurent, with his extraordinary sense of timing, gave challis it's biggest boost a year ago with printed separates and dresses. This year others have picked up the theme, sometimes teaming challis skirts with shirts and blazers, or as dresses for daytime or evening, frequently teamed with a big challis shawl.

Among the best of the challis designs this fall, from left:

Tiktiner teams up a wool challis plaid skirt with a flower-printed silk blouse and a big scarf also in challis.

Don Sayre of Gamut uses a huge fringed shawl to wrap up an empire waistline, paisley-printed dress, all in wool challis.

Giorgio Sant 'Angelo starts with a slit-sleeved peasant-style blouse in rayon challis for Gregory & Goldberg, then adds a sarong-shaped challis skirt to match.

A paisley-border print challsi is used by Albert Nipon for an overblouse dress cum shawl which can be tied as a head scarf.