can be used on textboxes to verify it the pressed key is a letter or not, usage: onkeypress = 'return isLetter(event)'

isLetter

function isLetter(evt) 
{
  evt = (evt) ? evt : event;
	var charCode = (evt.charCode) ? evt.charCode : ((evt.keyCode) ? evt.keyCode : ((evt.which) ? evt.which : 0));
	if (charCode > 31 && (charCode < 65 || charCode > 90) && (charCode < 97 || charCode > 122)) 
	{
		return false;
	}
	return true;
}