My book gives the following definition for reduced row echelon form: 1) If a row has nonzero entries, then the first nonzero entry is 1, called the leading 1 in this row. 2) If a column contains a leading 1, then all other entries in that column are zero. 3) If a row contains a leading 1, then each row above contains a leading 1 further to the left. 1 and 3 I understand, but 2 I don't fully understand - does that mean a 'column leading 1', as in if I start at the top of a column and go down the numbers then the first nonzero entry is a 1, or does it mean that if a column contains a 1 that is a leading 1 for its row? Thanks!