Circle to a Line is one of those calls that you learn in Mainstream one way, but you never ever dance it the way you learned it. The call is fraught with terror when square dancing with people that you've not danced with because you don't know whether they will dance its definition, or whether they will dance it the way every one else dances it....
Here's the definition:
CIRCLE TO A LINE: Starting formation - facing couples. TIMING - 8 steps.
Couples circle left one half (180°). The lead dancer in the couple who started on the inside (man's position) releases the left handhold, but retains the handhold of the dancer on his right to become the left end dancer of the line. The released dancer moves forward under the raised arm arch formed by that dancer and the adjacent dancer to become the right end dancer in the line.
STYLING: The circle portion is the same styling as in circle eight. As the man breaks with his left hand to form a line, he should lead the line several steps before turning. After the end lady has released her right hand, and while retaining the right hand of the man beside her, together they will raise their joined hands and make an arch. Then as the line begins to straighten out, she will move forward under this arch, turning left face gradually under her own left arm so that instead of backing up she is moving forward to the end of the line.
This call can be confusing. Although the definition states that the starting formation is facing couples, it is implicit in the movement of the call that one of the couples is a lead couple. If the dancers do not know who is the lead couple or who is the inside couple, they will not be able to perform the call. Most callers help out by preceding the call with a directed couple lead right, and that couple, then, is the lead couple.
Callers, help me out here, if this call were ever called from facing lines, is the lead couple that couple where the man is one of the centers of the line? It seems so from the definition, but I'm not sure. The big take away here is that you have to know whether you are the lead couple.
Of course, the biggest problem is determining whether the couple facing you is going to actually circle to a line, or do the shortcut. The shortcut is easy: the lead couple cross-folds around the other couple, and that couple moves left two or three steps. It isn't nearly as classy as the definition, but it gets you to the right place.
You should always know the definition, and be prepared to dance it. Most likely, though, you'll be dancing the shortcut. Of course, if you insist on dancing the definition at DC Lambda Squares, you may break down your square.