According to Success Criterion 1.3.1 (Info and Relationships), most input fields should be explicitly associated with a label. Technique H44, which describes the use of label elements, states that labels should be consistently placed for each type of input field, according to expected conventions.
For check-boxes and radio (option) buttons, labels should be placed after the field - in left-to-right writing languages, this means "to the right" of the input field. This example shows failures of this criterion, where the label has been placed after the element.