Geometric Shapes
Printed on:
• Portrait series all denominations
• 2015 SG50 $50 and SG50 $10 commemorative notes
Since 2008, Portrait notes signed by either Goh Chok Tong or Tharman Shanmugaratnam have included a new feature: small geometric shapes printed on the back. These shapes also appear on the 2015 SG50 commemorative notes. You will find one, two, or three shapes — squares, triangles, diamonds, stars, houses, or bells — in either solid or hollow form. The shapes can appear upright or inverted. They seem broadly linked to the prefixes, but there is no clear pattern. They are probably used for MAS’s internal tracking, and MAS has not explained how the shapes are chosen — which shape appears when, what each one means, or how they are sequenced. Most likely, they relate to the batch in which the notes were printed.
This list shows where every shape appears on the Portrait notes. (The $10000 note, issued before 2008, does not have these shapes.)
- Squares: One or two squares on all denominations except the $1000 note.
- Triangles: One or two triangles on all denominations.
- Diamonds: One or two diamonds on all denominations.
- Stars: One, two, or three stars on all denominations.
- Hollow stars: One, two, or three hollow stars only on $2 notes.
- Houses: One, two, or three houses on $2, $10, $50, and $1000 notes.
- Inverted triangles: One or two inverted triangles only on $2 and $10 notes.
- Hollow houses: One or two hollow houses only on $10 notes.
- Hollow squares: One or two hollow squares only on $10 notes.
- Hollow triangle: One hollow triangle only on $10 notes.
- Bells: One or two bells only on $100 notes. (The bells look like houses but have slightly rounded tops and sloping sides, so I classified them as bells.)
This list shows the first prefixes that correlate with each shape:
- $2: squares (3AG), triangles (4AA), diamonds (5AA), stars (6AA), hollow stars (6NA), houses (7AA), inverted triangles (9CA)
- $5: squares (3AA), triangles (4AA), diamonds (5AA), stars (6AA)
- $10: squares (2BF), triangles (3BA), diamonds (4BA), stars (4GL), houses (5EA/5BA), hollow houses (5NA), inverted triangles (6BA), hollow squares (7BA), hollow triangle (9BA)
- $50: squares (3AA), triangles (4AA), diamonds (4DY), stars (5AA), houses (6AA)
- $100: squares (1AA), triangles (2AA), diamonds (2AE), stars (3AA), bells (4AA)
- $1000: triangles (2AA), diamonds (3AA), stars (4AA), houses (5AA)
I’ve considered several theories to explain the geometric shapes on Singapore banknotes but have found inconsistencies in each. One theory is that the shapes follow a fixed sequence—squares, triangles, diamonds, stars, and houses—which fits the $10 and $50 notes but not others; for example, $2 notes include hollow stars before houses, $100 notes replace houses with bells, and $1000 notes begin with triangles, skipping squares entirely. Another theory suggests that the shapes match the prefix numbers, such as the $2 prefix 3 series having squares and prefix 4 series having triangles, but this too breaks down—$2 prefix 6 series includes both stars and hollow stars, $10 prefix 5 series shows both houses and hollow houses, and $100 prefix 2 series has both triangles and diamonds. I now believe the shapes are determined by a formula, similar to how NRIC numbers have a final letter calculated from the digits as a hidden verification tool. Likewise, the geometric symbols on banknotes may follow a hidden formula based on the prefix, batch, or other internal data. The Monetary Authority of Singapore has likely kept this system opaque to add an extra layer of authentication without revealing patterns that counterfeiters could easily copy.