I've got a picture frame which needs stringing on the back. 'D' rings at either side – cord run through each one and doubled to meet in the middle. What's the knot to tie to keep things tight?

Reef knot.
Or, if you can't tie a knot, tie a lot!

reef knot to join things, jesus whats happening to world, left over and under right over and under, its in the manual under "my first knot"

normal knot loose – thread other end through, tie that normal knot, then tighten them both. which is a crap explanation – worked with climbing ropes….

Use picture hanging wire and just twist it…

I would cut the cord to the width of the picture and then tie a round turn and two half hitches to the 'D' rings. Alternatively, you could tie the two ends in the middle with a reef knot but it won't be as tight!

Edit – The reef knot is in the book under 'My First Knot' but is not designed to be tight as it can be easily pushed apart.

Thread the cord through both d-rings then join the two ends with a "double fisherman's knot" This will form a continuous loop from which to hang you picture. You can also adjust this knot to make the loop smaller.

Siiiimmplez

Reef not is silly suggestion as it doesn't allow you to tighten anything.

I've used a basic tighening loop (don't know the name) on one end, and put the other end through it and back.

hows a reef knot going to be pushed apart with the weight at both ends you gimp? The weight of the picture will tighten it.

Dble fisherman's knot would be most effective for joining two ends of rope together, especially thin cord. More appropriate, than a reef knot, for this situation. IMO

Bowline on one end, pass the other end through the bight, tighten, and go thorough again, finish with a couple of half hitches.

Can't anyone do knots any more…

Surely the string will stretch over time. More so if its a heavy frame or large picture (so will need a longer string). If you don't tie a knot that you can use to shorten the string make sure that you can undo it easily enough when you come to shorten the string.

its only a picture, just tie a knot. And if the string is too loose just bang the nail in higher.

Use two hooks if its a heavy picture. One at each D ring.