| strong(adjective, hard, tough, strain, firm) |
| stable(adjective, secure, strain, firm, still) |
| severe(adjective, strong, hard, tough, correct) |
| sound(adjective, strain, firmness, labour, conformity) |
| consistent(adjective, strain, fixed, firmness, even) |
| unfluctuating(adjective, strain, still, fixed, firmness) |
| strict(adjective, good, strong, hard, tough) |
| insistent(adjective, determined, resolute, obstinate, uncompromising) |
| adamant(adjective, mean, strong, hard, tough) |
| obstinate(adjective, mean, hard, tough, moderate) |
| harsh(adjective, mean, strong, hard, tough) |
| unshakable(adjective, mean, hard, determined, unreasonable) |
| stern(adjective, hard, tough, easy, severe) |
| unbendable(adjective, firm, resist, determined, unreasonable) |
| rigorous(adjective, strong, hard, tough, correct) |
| tight(adjective, close, strong, secure, severe) |
| unalterable(adjective, severe, strain, harsh, cruel) |
| invariable(adjective, severe, strain, fixed, stiff) |
| refractory(adjective, hard, tough, strain, firm) |
| stationary(adjective, strain, fixed, firmness, labour) |