Sheldon says:
More and more rear wheels now are built "half-radial" with semi-tangent spoking on the right side and radial spoking on the left. Radial front wheels offer mainly esthetic benefits, but half radial rear wheels can be substantially more durable than conventional ones, in cases where the wheel is highly dished. The high amount of dishing called for to make room for more and more sprockets has caused an increase in spoke breakage on the left side of rear wheels. This is caused by metal fatigue.
A spoked wheel relies on having all of the spokes in constant tension. A highly dished rear wheel starts with very light tension on the left side spokes. The torque of hard pedaling combined with cyclical weight loading can cause the left side "leading" spokes to occasionally go completely slack momentarily.
Repeated cycles of tension and slackness cause these spokes to fatigue at the bends, and ultimately break.
With half-radial spoking, the amount of dish is very slightly less to begin with if you run the radial spokes up along the inside of its flange ("heads out.") In addition, since there are no "leading" spokes, no amount of torque on the hub can reduce the tension on any of the spokes. In fact, if you have an old wheel that has been breaking left side spokes, "half rebuilding" the wheel into a half radial will solve the problem once and for all.
I used to think that this was exotic, cutting edge technology, until I happened to look at a couple of Model A Fords in a local parade. Their wheels were highly dished inward, and were laced in the same half-radial pattern, for the same reason.
Wrong-way Half Radial
Sometimes, rear wheels are spoked half-radial with the radial spokes on the right. This is generally done for reasons of improving derailer clearance, particularly on wheels with unusually thick spokes or unusual flange designs. Such wheels require hubs with greater torsional stiffness since most of the driving torque must then be transferred by the left side spokes.
Seriously, youth of today. Before going to bikeforums.net, go to sheldonbrown.com ... that's how we did it back in the 2000s.