A massive wood purlin that runs parallel to the ridge of the roof about midway between the ridge and top plate: it is framed into the principal rafters and supports a number of common rafters.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved
In timber-framed construction, a purlin that is somewhat heavier than a common purlin; usually runs parallel to the ridge of the roof about halfway between the ridge and the top plate. The only purlin on each side of the roof ridge, it is framed into and joins the principal rafters, thus providing lateral stability for the entire roof framing system and support for a number of common rafters.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.