Polygraphic ciphers are encryption methods that encode multiple letters at once, rather than one letter at a time. This approach increases security by making it harder for attackers to decipher the message through frequency analysis, as patterns in the text become less obvious. Common examples of polygraphic ciphers include the Playfair cipher and the Vigenère cipher. These ciphers typically use a key or a grid to determine how letters are substituted or transposed. By employing a larger block of text, polygraphic ciphers can effectively obscure the original message, making them more complex than traditional monoalphabetic ciphers, which only replace single letters.