most common is 'one and a half'.some may say 'one and one half'.
When you use words like first,second,third,fourth,fifth...,you are actually saying their sequence.You don't say one and a half for placement.You can say something like 'this is between first and second'
This is because 1.5 is not the first,but it certainly places before second; therefore,by default,1.5 becomes second.Right?
one and a half is usually a value,seldom a placement.