Diploid cells abbreviated 2n and haploid cells abbreviated 1n because 'n' represents number of chromosome present in the cell. In haploid cells the number of chromosome is half in number while in diploid cells the number of chromosome is double so that's why haploid is abbreviated as 1n and diploid is abbreviated as 2n.
Diploid cells are present in body cells while haploid cells are found in sex cells.
