Collar beam
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A collar beam or collar is a horizontal structural member placed between two rafters to resist compression. Common in domestic roof construction to prevent rafter sag, a collar beam is often colloquia
Roof framing: element #5 is the collars
An old collar beam roof in the Netherlands
Rafter
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A rafter is one of a series of sloped structural members such as steel beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof
Common rafters without collar beams form most of this roof. There is not always a ridge board or beam where the rafter tops meet. Under the midsections of the rafters are purlins which support the common rafters and are supported by principal rafters. This roof ends in an octagonal hip.
Rafter and tie-beam joints (Carpentry and Joinery, 1925)