B1 and B2 are the third and fourth of six language levels on the Common European Framework of Reference for Languages, or CEFR. The CEFR is an international guideline to measure language skills, using a scale from A1 (beginners) to C2 (language masters). According to the IELTS website, B1 corresponds to IELTS scores from 4-5, and B2 corresponds to scores 5.5-6.5.