Made from plastic, these reducing Y pieces have a "stepped" construction and can be cut to the exact size you need. Available in three different models, all models have one side that is bigger than the other two:-
Y1 - The smallest of the range, the larger hole measures 200mm/8" and the two smaller holes measure 150mm/6".
Y2 - The mid size model, the larger hole can be used as either 250mm/10" or 200mm/8" and the two smaller holes can be used as either 200mm/8" or 150mm/6".
Y3 - The largest model, the larger hole can be used as either 300mm/12"(fits a 315mm OK) or 250mm/10", the two smaller holes can be used as either 250mm/10" or 200mm/8".
It's worth mentioning that whilst these products look expensive, when you compare them to the cost of their metal equivalents they are in fact slightly cheaper and also offer the flexibility of being able to change sizes.