Our ancestors treated learning with reverence and respect, possibly because most of the teaching, three and four centuries ago, was performed by men trained for the ministry.
So it is not amazing to find that when children learned the alphabet, the little “hornbooks” from which they studied were almost invariably decorated with a cross.
Sometimes there was just one cross, preceding the letter “A.” Sometimes there was one at the beginning of the alphabet and another at the end, and sometimes the alphabet itself was arranged in the form of a cross.
The cross was itself referred to as Christ-cross, to distinguish the figure from the letters that followed, and the row of letters forming the alphabet came to be known as Christ-crossrow. Along with the pronunciation of Christmas, Christian, Christopher, Christ-cross was always sounded criss-cross, and was often so spelled.
Ultimately, in this form, it took on the special meaning which we now give it, a series of crossing lines.