CORE packs are based on the most common words in the English language. It is
a great place to start if you are learning English, because it will
consolidate your vocabulary at the same time as training your spelling. Each
pack contains 750 words, so if you complete Core 1 and Core 2, you will be
familiar with the 1500 most common English words.

The CORE packs have been created by collating a series of word frequency
lists. These include written and spoken sources, and we have been careful in
our selection to best reflect what we think is the most relevant and best
reflects modern usage of English.