The rule is that units named by persons use a capital letter (Kelvin, Tesla, Newton, others use a small one (meter, gram). According to the rule liter was desig

The rule is that units named by persons use a capital letter (Kelvin, Tesla, Newton, others use a small one (meter, gram). According to the rule liter was designated by a small l. In 1979 the capital L was recognized by SI (as non-SI unit), too. (I don't know whether the script form was recognized at the same time). According the German Wikipedia capital L is used mainly in English speaking countries. Which makes sense because in handwriting the American (not so sure about the British) digit 1 and small letter l cannot really be distinguished. So 1 liter written as 1 l would be hard to read. In Germany the digit 1 nearly unviversally has a rising stroke on the left in handwriting, so 1 l is not a problem. German Wikipedia says capital L is not used by ISO.