 While "slimline" is not a defined term, slimline seats have less padding in the back.Seat pitch and width in economy class have also been decreasing, In 1985 none of the main four US carriers offered a seat less than 19 inches wide.Since the beginning of the twentyfirst century until 2018 average seat width decreased from 18.5 to 17 inches, and sometimes as low as 16.1 inches.