Before the number, just know that the triangle too conveys a message, about recyclability of Plastic material. The number insider triangle indicates the type/ category of Plastics which should be kept together before subjecting it to melting and recycling. Not all Plastics are the same and one type of Plastic when mixed with another type, might either spoil the whole mix or mostly it would remain incompatible. So in simple words, any plastic should be checked for its recyclability number and should be kept together before melting together for recycling.