Our dining table is 48"x48" square, which is actually fairly irregular. We had it custom made from Room and Board, sized to be the perfect board game table–and it is! The only problem is, it's difficult to find square tablecloths that fit. Rectangular ones are both too long and too narrow, and circular ones look a little bizarre on the square.
I did a few quick searches, and found that standard table widths are around 35"-40" wide, and tablecloths drop around 10", give or take a few inches. So if I want a 12" drop on each side of my table, I would need a tablecloth that is 60"x60".
I've had this blue and silver batik fabric around for quite a while; I had bought it two years ago at Christmas as a furoshiki cloth, and I think I used it for that purpose just once. At 44" wide, however, it was still too narrow, but I found a mostly matching blue cotton in my stash to fill out the ends. A bit of measuring and cutting (OK, I got lazy and ripped. At least I was on grain as a result!), and I was ready to sew.
Sewing the ends together
Trimming to move the seam allowance under the more opaque solid blue.
Topstitching with some silver thread I found.
Add a simple hem to the edges and voila!