Rail Fence

The rail-fence cypher is a transposition cypher that takes as a key a number of rails (in this example $3$ ). The plain-text message is written on the diagonal, going up and down on the rails:

G...D...O...E....
.O.D.A.H.W.R.Y.U.
..O...Y...A...O..

Finally, the message is read-out horizontally:

GDOE ODAHWRYU OYAO

Knowing the number of rows, we can write each segment under the other:

GDOE
ODAHWRYU
OYAO

and then reconstruct the rail-fence in order to obtain the plaintext.