The 10–20 system: measure, derive, place

Answer, then get the reasoning. You can leave at any point — your place, your answers and your review schedule are saved. Nothing here is timed.

Work through it

One decision per screen. Decide before the reveal; confidence is recorded before the answer.

Step 1 of 6

14 min

Three landmarks do all the work

The 10–20 system does not measure in centimetres from a fixed point. It measures the head, then takes percentages of that measurement. That is why it works on a 34 cm head and a 60 cm head.

Nasion — the depression at the bridge of the nose.
Inion — the bony prominence at the back of the skull.
Preauricular points — the notch just in front of each ear.

In plain words

Nasion: The depressed area between the eyes at the top of the nose; the anterior sagittal landmark.
Inion: The external occipital protuberance; the posterior sagittal landmark.

Explain this simply

You are dividing the head into proportions instead of guessing. Measure front-to-back and ear-to-ear, take fixed percentages of each, and the sites land in the same relative place on every head — big, small, adult or child.

Why am I learning this?

Objectives attached to this activity.

Identify nasion, inion and preauricular points and explain why proportional measurement produces reproducible sites.
Developing · 71%
Every study you record, and every comparison between studies, depends on the sites being where they claim to be.
Prevents: Mis-measured sites shift apparent lateralization and make serial studies non-comparable.
comprehension
Derive the standard 10–20 positions from measured landmarks and place them on a head diagram.
Developing · 65%
You place these sites on every patient, often in poor conditions at the bedside.
Prevents: Guessed placement is the most common source of unusable serial comparisons.
application
Evaluate electrode integrity and patient comfort during application and correct problems before recording begins.
Secure · 85%
A study that degrades at minute 4 costs a repeat and a second patient visit.
Prevents: Prevents repeat studies caused by an electrode that was never going to hold.
application

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