Reduced row echelon form question

[wumple]

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!

 

[Landau]

No, it means a "row leading 1", as defined in 1). So, if a ROW has a leading 1, than in the COLUMN of that particular leading 1 all other entries are zero. Perhaps the wkikipedia definition is clearer.

 

[wumple]

Thank you!