SINGLE

One layer of petals surrounding prominent central cluster of stamens (in which odd petaloids may appear).

an example

Tama-No-Ura

SEMI-DOUBLE

Two or more layers of petals surrounding usually prominent central cluster of stamens (in which odd petaloids may appear).

an example

Great Eastern

DOUBLE

Large outer petals surrounding distinct central crown, cushion or cluster of bunched petals or petaloids (Waratah form).
 
an example

Elegans Champagne

INFORMAL DOUBLE

Double bloom comprising several layers of petals mostly twisted, irregular or confused and mostly obscuring stamens.
 
an example

Lady Loch

FORMAL DOUBLE

Of regular form - many layers of petals arranged symetrically to bud centre. Formal doubles are not suitable for salt laden winds in coastal positions.
 
an example

Prince Frederick William

Bloom Colour variation where mentioned (e.g. "bright to deep red") allows for varying soils, localities and temperature changes during the flowering period.