Chromotropic stitching threads are threads which have one colour in daylight and a second colour under UV light.

Those threads are usual in security books and offer a third level of security, more sophisticated than threads which are visible and fluorescent.

Examples of those threads are:

• visible blue in daylight and fluo green or red under UV light,
• visible green in daylight and fluo yellow or red under UV light,
• visible yellow in daylight and fluo blue or green or red under UV light,
• visible orange in daylight and fluo green or red under UV light,
• visible red in daylight and fluo blue or green under UV light,
• visible violet in daylight and fluo blue or red under UV light.