Credit: Moore & Merrill (1968, p. 32). Copyright status uncertain. The image may be public domain: it's from a US government publication that is available online, but one author was not a US government employee. This image may fall in the-no-one-is-aggrieved category. If anyone considers the use here inappropriate, I will resolve the matter promptly.

Description of Grotrian diagram following Moore & Merrill (1968, p. 3).

1. The diagrams are often highly simplified omitting many terms and multiplets. One hopes the lowest terms and strongest multiplets are nearly complete.

2. The indicated atomic divisions are the atomic terms.

3. The energy scale is in wavenumber in inverse centimeters on the left and eV on the right.

4. Allowed multiplets are connected by solid lines.

5. Semi-forbidden multiplets are connected by dashed lines.

6. Forbidden multiplets are connected by dot-dashed lines.

7. Metastable levels are those connected to all downward levels by forbidden lines or no lines. One hopes this is always true.