Signed with a mark

Marriage certificates in Great Britain are great! I love that they include the bride and groom's signatures.  How cool is it to see our ancestor's handwriting?!  However, there is one exception: when they sign with their mark.  What is that?

Let's check out two examples from my family that are 100 years apart.

1871

In 1871, my 2x great-grandfather signed his own name, John Boyle.  But his new bride wrote an X and the clergyman wrote "The mark of Catherine O'Neill".

Family member name: Catherine O'Neill Boyle

Lifetime: 1849 - 1941
Tree branch: Boyle
Places lived: Durham, England; Indianapolis, Indiana

Relation: My 2x great-grandmother (my mom's dad's dad's mom)

1771

In 1771, my 5x great-grandparents, William Fripp and Betty Knight both added their marks with an X.  

Family member name: Betty Knight

Lifetime: 1744 - 1831
Tree branch: Bromley
Places lived: Dorset, England

Relation: My 5x great-grandmother (my dad's mom's mom's mom's dad's dad's mom)

Why?

Why did they sign with a mark?  There are multiple theories.

• Most likely, the participant could not read or write. Illiteracy was very common prior to 1875 in Great Britain. On the other hand, we can't really assume that those who signed their name were completely literate, as some could write their name, but that's all.
• They were simply asked to put their mark by the official in charge of the ceremony.  Maybe the official assumed they were illiterate.
• You never know, they could have been blind!