Abstract: A golf club putter head affixing an elongate spirit level may be tilted relative to earth and rested on its two frustaconical projections so as to accurately measure earth inclination nonetheless that, inter alia, the sole of the putter head is curved, and thus unsuitable to serve to reference the putter head to the earth's surface. The two frustaconical projections are preferably spaced parallel, equal length, maximally separated and axially weighted. The spirit level is preferably both removably affixed--permitting use of the putter in regulation play--and of maximum elongate extent. A preferably offset hostel is affixed in a position preferably substantially midway between maximally-separated spaced-parallel frustaconical projections.