Why are these beds described by modern mattress size?

Because the frames have been widened or re-railed to take contemporary bases. A dealer writes king, super king, double or single so the buyer knows what will fit. The language is standard across the archive. Some form of king appears in 101 of 330 titles, against 51 doubles and 43 singles.

Are all painted antique beds French?

No, though French dominates. Origin is stated as French in 175 of 330 titles, and the French frames make up the bulk of the sold archive. Italian, Portuguese, Irish, Spanish, Scandinavian, Austrian, Dutch-style and American-style examples all appear in the sampled titles. Painted sheet metal, toleware, shows up only twice, once Austrian and once Italian.

What does original paint mean on a bed?

A surviving old painted surface rather than a modern repaint. Listings use "original paint" and "aged paint" for it, the latter in ten titles. The alternative is a newly painted frame, sometimes offered in a custom colour chosen by the buyer. Both sell, but the two descriptions sit in different parts of the range.

How old are painted beds usually?

Dealer-supplied dates run 1850 to 1930 across 254 listings with a clean year. The 1920s account for 88 of those, the 1900s 76, so the early twentieth century carries the great majority. Another 43 listings carry period words instead of dates, mainly "19th Century" and "Victorian". A few titles claim Regency or 18th century, and those attributions are loose.

Can a pair of single beds be joined?

Several listings say so. Pairs are a standing format, mentioned in 35 titles, and some specify the two singles can be pushed together into one wide bed. Matching pieces turn up alongside them: bedsides, chairs, occasionally a full bedroom suite.

What are caned and bergère painted beds?

Caned beds have head and foot panels of woven cane set into the painted frame. Bergère describes the curved, enclosing version, a term taken from chair forms. The combination is common enough to have its own title formula, French painted caned bergère at kingsize, and these sit in the upper part of the sampled range.