I've always had a weakness for orphaned cups. The tea and coffee cups I sell are often 200 years old, sometimes more. So it is not surprising that some have lost their siblings. But even so, they can be perfect for use, and they make great gifts!
Many of these cups would not only have had a saucer, but also a companion cup. In the 18th and early 19th Century, cups and saucers were sold in "true trios": a teacup, a coffee cup and a saucer. As you would never drink tea and coffee at the same time, why invest in an extra saucer? People would be offered a choice of tea or coffee after dinner, so they could choose which cup they wanted, and the other cup would be taken away.