Abstract
Twenty-one centimetre neutral hydrogen measurements of NGC 4151 show that the position angle of its major axis is 26° ± 3° rather than the generally adopted value of ∼ 130° derived from the brighter isophotes. Also the inclination is 26° ± 8° which makes it more face-on than previously believed. Neutral hydrogen extends ∼9′ arc along the major axis and has a normal surface density for a Sab galaxy in the regions where the outer optical spiral arms are extremely faint. Some implications of these results are discussed.

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