Large-format line arrays are designed for large venues or outdoor festivals. These boxes typically included multiple vertically aligned high frequency compression drivers and multiple midrange and low drivers arranged symmetrically around the compression driver. The low frequency driver is typically 15 or 18 inches in diameter. Mid-format line arrays are typically two or three way and use 10 or 12 inch low-frequency drivers. The horizontal coverage is typically 90 degrees wide but some systems employ narrower boxes at the top or wider boxes at the bottom of the array. Using a transition frame (which aligns the rigging on dissimilar systems), system engineers may sometimes hang a mid-format box below a large-format box to cover the closest audience members. Speaker boxes from different manufacturers are not mixed because each system has a particular 'voicing' which may be common to a single manufacturer.