awk 'NR % 2 == 1' // every 2nd line
awk 'NR % 10 == 1' // every 10th line
awk 'NR % 100 == 1' // every 100th line

NR ordinal number of the current record<br/ > <br/ > X == 1 keeps the first line intact, whereas X == 0 would remove the first line. Important for CSV files.