Combination spanners have two heads: one with an open-ended profile and the other with a ring profile. See What is an open-ended spanner? and What is a ring spanner? for more information on these profile types.
The two heads are usually the same size and range from 4mm to 50mm or 5/32″ to 2″. The length of the spanner increases with head size from about 100mm (4″) to 500mm (19½”).
Combination spanners are good value for money as you get two tools for the price of one although remember that both the heads are the same size so you only get one size per spanner.
The ring profile can be non-ratcheted or ratcheted and sometimes it is flexible (see What is a ratchet spanner? and What is a flex-head spanner? for more information).
The open-ended head is stronger than the ring head style because it puts less pressure on the corners of the fastener, the weakest points on the fastener head. Because of this, the open-ended head is perfect for the final tightening of a fastener or for the first few turns of loosening one. The ring head can then be used for the main part of the task.
One combination spanner design has an open-ended head with a few modifications to the standard style. The profile is star-shaped with a plastic component on one face of the head making the profile the common U-shape.
This means that depending on how the tool is fitted onto the fastener, it will either slip over it or the plastic component will catch and ‘lock’ it in place, making it less likely to slip of the fastener. This makes it very easy to remove and replace the tool when turning the fastener because less care is needed to prevent the tool slipping.