A UK half treble is so called because you are working the initial stages of a treble, but the execution of the stitch is shorter.
In the US, a half treble is known as a half double, so bare this in mind when you are reading patterns and make sure you are working the stitch specified, by identifying whether your pattern is using US or UK abbreviations.
How to crochet a half treble
In height, the half treble is halfway between a double crochet and a treble and is frequently used as a transitional stitch between the two in waves and other curvy patterns.
1. When working into a chain, begin by wrapping your yarn round the hook and insert the hook in the third chain from the hook. This gives the stitch necessary height where the two chains just missed and will later be counted as a stitch. 2. Yarn round your hook and pull the yarn through the chain to make a loop on the hook, then wrap your yarn round the hook again. 3. Pull the yarn loop just made through all three loops on the hook to complete the stitch.
If working a UK treble, at stage three (above), there are two steps instead of one - wrapping your yarn round the hook and bringing it through two loops twice. This elongates the stitch, creating a taller one.
As mentioned earlier, the half treble is slightly taller than a double crochet and slightly shorter than a treble. So often this will be used as an incremental stitch in between the two when working shell-like shapes.
In the above pansy, half-treble stitches are used alongside longer treble stitches to add shapes to the existing curves formed naturally by working in the round.
This pansy is worked around the centre which you can see is worked in a cheery bright yellow.