| firm(noun, adjective, feature, state, characteristic) |
| consistent(adjective, strong, strain, flat, certain) |
| strong(adjective, tough, strain, flat, firm) |
| stable(adjective, strong, tough, strain, flat) |
| hard(adjective, feature, state, characteristic, strain) |
| sound(adjective, strain, firmness, labour, hardiness) |
| stiff(adjective, feature, state, characteristic, strain) |
| unfluctuating(adjective, strain, steady, regular, firmness) |
| constant(adjective, strong, tough, continue, strain) |
| steadfast(adjective, feature, state, characteristic, strong) |
| steady(adjective, feature, state, characteristic, strong) |
| enduring(adjective, quiet, strong, tough, continue) |
| perpetual(adjective, continue, strain, determined, usual) |
| lasting(adjective, strong, tough, continue, strain) |
| permanent(adjective, strong, tough, continue, strain) |
| unyielding(adjective, strong, tough, strain, flat) |
| durable(adjective, continue, strain, stable, firmness) |
| rigid(adjective, feature, state, characteristic, severe) |
| invariable(adjective, strain, certain, stable, regular) |
| lingering(adjective, continue, strain, firmness, labour) |