if the Master of Ceremonies at a ball or Assembly so decreed.....but only i special circumstances:

Two Ladies , or two Gentlemen, cannot dance together, without permission of the Master of Ceremonies; nor can permission be given while there are an equal number of Ladies and Gentlemen.

In the absence of Gentlemen, when ladies are permitted to from couples, and in the absence of Ladies, when it occurs that Gentlemen are permitted to form couples, they must always stand at the Bottom of the Set.

Page 264,The Complete System of English Country Dancingby Thomas Wilson