This is a longer stitch than treble crochet, created by adding an extra yarn over at the start of the stitch. This is a UK double treble, which is the same as a US treble, for a handy conversion chart, click here
Double treble crochet (Dtr)
Yarn over twice
Insert the hook into the centre of the chain,
Pull through 2 (3 loops on hook)
pull through 2 loops (2 loops left on hook).
pull through 2 loops (1 loop left on hook).