Abbreviations in English 1

Is there an actual system behind English abbreviations, or is it just memorization? Mostly a system, though not one single rule. Most abbreviations fall into a few recognizable patterns: clipping a word down to its first part (Doctor → Dr., December → Dec.), using a standard unit symbol (inches → in., teaspoons → tsp.), or combining the first letters of each word in a phrase (United States → US). A handful of abbreviations are genuine historical exceptions, though: lb for pound comes from the Latin word libra, which is why it doesn't start with an L the way you'd expect. Those ones just have to be memorized individually.