If you look against the light at the front of the 100-, 200-, 500- and 1000-peso notes, you can see a thread about one millimeter in width that crosses the note vertically. This thread is known as the security thread and is located almost immediately to the left of the bust-head figure.

There is another thread that also crosses the note vertically, known as the micro-printed thread. In this thread you can see printed the note's denomination. On the front of the 100- and 500-peso notes, this thread is discontinuous and it is located to the left of the bust-head figure, before the start of the perfect register. On the front of the 20-peso notes, it is located next to the image of the dove. On the front of the 50-peso notes, it is found next to the image of the butterfly.

In the case of the 200- and 1000-peso note, there is also a thread known as 3D thread, which is found to the left-hand side of the note (as seen from the front). This thread has prominent parts where you can see a figure of a pre-Hispanic snail, related to the God of the Wind, Ehécatl. This figure moves when you tilt the note sideways, backwards, and forwards. The figure moves in sense opposed to the turn of the note. If you place the note against the light, you can see a band across it. The 3D thread is green in the 200-peso notes and violet in the 1000-peso notes.

In general, the threads are part of the paper or polymer from the time of its manufacture.

To observe these security features (except in the case of the 3D thread), place the note against the light.