Card Blank sizes are a little more confusing. Instead of the measurement representing the size of the entire sheet of card, the measurement represents the size of the card blank when it's folded (the card's 'front face'). So, an A5 card blank refers to the size of the folded card – it measures an A5 size – whereas the size of the card when unfolded is actually a size bigger – A4. An A6 card blank is the folded size, while the flat size is A5, and so on.
Square card blanks are measured in centimetres or inches, while DL card blanks have one set size: 21cm x 9.9cm (8.3" x 3.7") – they are a third of an A4 sheet of card.