Can you give us any references or citations to differentiate the two and clarify what precisely you are talking about I m finding it difficult to find any reference to straight two at all except those referencing the Wikipedia On the other hand I can find plenty that use parallel twin for 180 270 or 360 degree twins and referred to it as most commonly known If you read the references I gave you will also see it is widely used for automobiles snowmobiles ATV jet skis and so on and has been for decades Even if you can not provide references I d genuinely like to know where and from when the term was used What I think you are suggesting is that parallel twin is used only for 360 degree engines but that is just not so