Counterbore symbol

A flat-bottomed bracket shape, calling for a flat-bottomed enlarged hole.

Also known as: Spotface symbol, Square bracket on a drawing, U shape drawing symbol

Engineering & technicalStandard profile

Meanings

A square-bottomed bracket calls for a counterbore: a flat-bottomed widening so a bolt head sits below the surface. Its near-twin, the countersink, is a V — and the shape of the symbol is a picture of the hole.

Common modern usageVerified fact
Calls for a flat-bottomed cylindrical enlargement of a hole, so a bolt head or nut sits at or below the surface.
Contexts: Technical drawing
Common modern usageVerified fact
Is drawn as a picture of the resulting hole in section, which is why the counterbore is square-bottomed and the countersink a V.
Contexts: Technical drawing

Visual description

Two short vertical strokes joined at the bottom by a horizontal one, forming a squared-off U open at the top.

Symmetry: vertical
Orientation: upright

Commonly confused with: Countersink symbol
Both are small bracket-like marks before a hole dimension; square bottom against V bottom.

Compare: Counterbore vs countersink →

Confidence

Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references, but multiple independent sources agree.
IdentityHigh confidence: High confidence because the identity is documented in encyclopedic references, but multiple independent sources agree.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but multiple independent sources agree, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is documented in encyclopedic references, but multiple independent sources agree.

Sources

Counterbore
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.
Countersink
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.
Technical drawing
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.

Last reviewed 2026-07-31

Part of

Technical drawing notation

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Meanings

Common modern usageVerified fact

Calls for a flat-bottomed cylindrical enlargement of a hole, so a bolt head or nut sits at or below the surface.

Contexts: Technical drawing

Common modern usageVerified fact

Is drawn as a picture of the resulting hole in section, which is why the counterbore is square-bottomed and the countersink a V.

Contexts: Technical drawing

Confidence

Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references, but multiple independent sources agree.
IdentityHigh confidence: High confidence because the identity is documented in encyclopedic references, but multiple independent sources agree.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but multiple independent sources agree, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is documented in encyclopedic references, but multiple independent sources agree.

Sources

Counterbore
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.
Countersink
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.
Technical drawing
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.

Last reviewed 2026-07-31

Meanings

Visual description

Related & commonly confused symbols

Meanings

Visual description

Related & commonly confused symbols