"No lady should cross her knees so that her skirts go up to or above them; neither should her foot be thrust out so that her toes are at knee level. An arm a-kimbo is not a graceful attitude, nor is a twisted spine! Everyone, of course, leans against a chair back except in a box at the opera and in a ballroom but a lady should never throw herself almost at full length in a reclining chair or on a wide sofa when she is out in public...The proper way for a lady to sit is in the center of her chair, or slightly sideways in the corner of a sofa. She may lean back, her hands relaxed in her lap, her knees together, or if crossed, her foot must not be thrust forward so as to leave a space between the heel and her other ankle. On informal occasions she can lean back in an easy chair with her hands on the arms."
Emily Post ~ Etiquette in Society, in Business, in Politics and at Home