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As a historical curiosity, I should note that even once adopted, not everybody used precisely the same symbol for +. Widman himself introduced it as a Greek cross + (the sign we use today), with the horizontal stroke sometimes a bit longer than the vertical one. Mathematicians such as Recorde, Harriot and Descartes used this form. Others (e.g., Hume, Huygens, and Fermat) used the Latin cross "✝," sometimes placed horizontally, with the crossbar at one end or the other. Finally, a few (e.g., De Hortega, Halley) used the more ornamental form "✠." (责任编辑：) |