A recent survey of over 1000 palindromes has shown that 71 per cent pivot on a single letter, as in SEXES, EVE and MESEM, whereas only 29 per cent have a double letter in the centre, as in TIBBIT, OPPO and REDDER. From the palindromes with an even number of letters I have selected 26, starting with AA and working through the alphabet to ZZ.
"Palindromes with Central Pairs,"
Word Ways: Vol. 15
, Article 19.
Available at: http://digitalcommons.butler.edu/wordways/vol15/iss1/19