Polyalphabetic ciphers use more than one replacement scheme.
The letter of the key word is located in the first row and the plain text letter is located in the first column. To find the substitution letter, trace the column of the key letter down to the row of the plain text letter and use the letter found in this cell. The plain text letter is then found at the first column of that row. There are three possibilities to use the keyword. The substitution letter is found in the first row of this column.