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Digital watermarking is widely believed to be a valid means to discourage illicit distribution of information content. Digital watermarking methods for text documents are limited because of the binary nature of text documents. A distinct feature of a text document is its space patterning. We propose a new approach in text watermarking in which interword spaces of different text lines are slightly modified. After the modification, the average spaces of various lines have the characteristics of a sine wave and the wave constitutes a mark. Both nonblind and blind watermarking algorithms are discussed. Preliminary experiments have shown promising results. Our experiments suggest that space patterning of text documents can be a useful tool in digital watermarking.