The opening or clearance gap between the spinning sheave (wheel) and the outer shell (cheeks) of a rigging block. The swallow must be wide enough to allow the specified diameter of line to run freely without chafing, but tight enough to prevent the line from jumping off the sheave and jamming.
Heard on the job
We need a block with a larger swallow for this halyard, the line is chafing against the cheeks.
Insider tip
A common mistake is using a line that barely fits the swallow; once the line swells with salt water, it will bind and jam the block.