The use of outline in embroidery hardly needs pointing out. It is often the obvious way of defining a pattern, as, for example, where there is only a faint difference in depth of tint between the pattern and its background; in applique work it is necessary to mask the joins; and it is by itself a delightful means of diapering a surface with pattern of a kind which is never obtrusive.
Allusion to the stitches suitable to outline has been made already (see Stitch-groups), as well as to the colour of outlining (a propos of applique). It is difficult to overrate the importance of this question of colour in the case of outline ; but there are no rules to be laid down, except that a coloured outline is nearly always preferable to a black one. The Germans of the sixteenth century were given to indulging in black outlines, and you may see in their work how it hardened the effect, whereas a coloured outline may define without harshness. The Spaniards, on the other hand, realised the value of colour, and would, for example, outline gold and silver upon a dark green ground in red, with admirable effect. A double outline, for which there is often opportunity in bold work, may be turned to good account. Among the successful combinations which come to mind is an applique pattern in yellow and white upon dark green, outlined first with gold cord, and then, next the green, with a paler and brighter green. Another is a pattern chiefly in yellow upon purple, outlined first with yellow couched with gold, and next the ground with silver. In the case of couched cord or gold, the colour of the stitching counts also.
Stitches from the edge of a leaf, inwards, alternately long and short, though they form an edge to the leaf, are not properly outlining This is rather a stopping short of solid work than outlining, though it often goes by that name.
The first condition of a good outline stitch is that it should be, as it were, supple, so as to follow the flow of the form. At the same time it should be firm. Fancy stitches look fussy; and a spiky outline is worse than none at all.
There is absolutely no substantial ground for the theory that outlines should be worked in a stitch not used elsewhere in the work. On the contrary, it is a good rule not to introduce extra stitches into the work unless they give something which the stitches already employed will not give. The simplest way is always safest.